Method to prepare composite graphite particles for nonaqueous secondary battery negative electrode, active material for nonaqueous secondary battery negative electrode, and nonaqueous secondary battery

ABSTRACT

The invention relates to composite graphite particles for a nonaqueous secondary battery negative electrode containing graphite (A) and metal particles (B) capable of alloying with Li, wherein the degree of dispersion of the metal particles (B) in the composite graphite particles is 0.78 or more and the internal void fraction of the composite graphite particles is 3% or more and 40% or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of prior U.S. applicationSer. No. 16/866,688, filed May 5, 2020, now allowed; which is acontinuation application of prior U.S. application Ser. No. 16/159,457,filed Oct. 12, 2018, now U.S. Pat. No. 10,680,238, issued Jun. 9, 2020;which is a continuation application of prior U.S. application Ser. No.15/275,824, filed Sep. 26, 2016, now abandoned; which is a continuationapplication of PCT/JP2015/059287, filed Mar. 26, 2015; all of whichclaims priority to Japanese Application No. 2014-064534, filed Mar. 26,2014, the disclosures of which are incorporated herein by reference intheir entireties.

TECHNICAL FIELD

The present invention relates to composite graphite particles for anonaqueous secondary battery negative electrode, to an active materialfor a nonaqueous secondary battery negative electrode that uses thoseparticles, and to a nonaqueous secondary battery provided with anegative electrode containing this negative electrode active material.

BACKGROUND ART

The demand for high-capacity secondary batteries is growing accompanyingthe reduced size of electronic devices. Nonaqueous secondary batteries,and particularly lithium ion secondary batteries, which demonstrate highenergy density in comparison with nickel-cadmium batteries ornickel-hydrogen batteries, are attracting particular attention. Lithiumion secondary batteries, which are composed of positive and negativeelectrodes capable of occluding and releasing lithium ions and anonaqueous electrolytic solution obtained by dissolving a lithium saltsuch as LiPF₆ or LiBF₄, have been deployed not only in conventionallaptop personal computers, mobile communications devices, portablecameras and handheld game consoles, but also in power tools and electricautomobiles, thus resulting in a growing need for higher capacities,faster charging and discharging characteristics and higher cyclingcharacteristics in lithium ion secondary batteries accompanying theirdeployment in such applications.

Although various types of materials have been proposed for the negativeelectrode material of these batteries, natural graphite, artificialgraphite obtained by graphitization of coke and the like, graphitizedmesophase pitch and graphitic carbon materials such as graphitizedcarbon fiber are used at present due to their high capacity and thesuperior flatness of their discharge potential.

In recent years, studies have been conducted on the application ofmaterials having high theoretical capacity, and particularly metalparticle negative electrodes, with the aim of further increasingcapacity.

For example, Patent Documents 1 and 2 propose methods for producingcomposite Si-graphite particles by firing a mixture of fine particles ofan Si compound, graphite and a carbonaceous precursor in the form ofpitch and the like.

Patent Document 3 proposes composite Si-graphite particles obtained bycompounding Si fine particles with a carbonaceous material so that theSi fine particles are unevenly distributed on the surface of sphericalnatural graphite.

Patent Document 4 proposes composite graphite particles having as maincomponents thereof metal capable of alloying with Li, flake graphite anda carbonaceous material, wherein the metal is held between a pluralityof flake graphite layers, and specifically discloses compositeSi-graphite particles.

Patent Document 5 proposes composite graphite particles composed of agranulated body obtained by crushing and granulating a mixture of agraphite raw material and metal powder in a high-speed airflow, whereina portion of the graphite serving as raw material is crushed and thegraphite raw material and its crushed product aggregate to form alaminated structure in which metal powder is present in a dispersedstate internally and on the surface thereof, and specifically disclosescomposite Si-graphite particles.

Patent Document 6 discloses composite Si-graphite particles composed ofroughly spherical particles having carbon microprotrusions on thesurface thereof obtained by granulating and spheroidizing a mixture ofvoid forming agents selected from vein and/or flake natural graphite,fine particles of an Si compound, carbon black, polyvinyl alcohol,polyethylene glycol, polycarbosilane, polyacrylic acid andcellulose-based polymers, and impregnating and coating the resultingspherical granulation product with a mixture of a carbon precursor andcarbon black followed by firing.

Patent Document 7 discloses composite Si-graphite-particles of a form inwhich Si particles are sandwiched between flake graphite, and areobtained by applying compressive force and shearing force to a mixtureof Si particles, flake graphite and a solid non-graphitic carbon rawmaterial at a temperature equal to or higher than the softening point ofthe non-graphitic carbon raw material to prepare intermediate compositeparticles followed by heat treatment.

Patent Document 8 discloses composite Si-graphite particles having astructure in which graphite is folded by mixing Si particles and flakegraphite and subjecting to spheroidizing treatment.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Publication No.2003-223892

Patent Document 2: Japanese Unexamined Patent Publication No.2012-043546

Patent Document 3: Japanese Unexamined Patent Publication No.2012-124116

Patent Document 4: Japanese Unexamined Patent Publication No.2005-243508

Patent Document 5: Japanese Unexamined Patent Publication No.2008-027897

Patent Document 6: Japanese Unexamined Patent Publication No.2008-186732

Patent Document 7: International Publication No. WO 2013/141104

Patent Document 8: International Publication No. WO 2014/046144

DISCLOSURE OF THE INVENTION Problems to Be Solved by the Invention

However, according to studies conducted by the inventors of the presentinvention, in the art described in Patent Document 1, since compositeSi-graphite particles obtained by compounding graphite and Si compoundparticles with a carbonaceous material exhibit weak bindability of thecarbonaceous material responsible for binding the composite Si-graphiteparticles, the composite Si-graphite particles collapse due to volumeexpansion of the Si compound particles accompanying charging anddischarging, thereby preventing this art from reaching the level ofpractical application due to problems such as cycle deteriorationattributable to interruptions in conductive paths and the like.

In the art described in Patent Document 2, although a structure isproposed (by defining the range of the Raman R value) in which thesurfaces of Si compound particles and the surfaces of flake graphiteparticles are covered by amorphous carbon as a result of adequatelystirring and mixing composite Si-graphite particles containing Sicompound particles, flake graphite particles and a carbonaceous materialderived from coal tar pitch prior to compounding (firing), since thebindability of compounding is weak in this case as well, the compositeSi-graphite particles collapse due to volume expansion of the Sicompound particles accompanying charging and discharging, therebypreventing this art from reaching the level of practical application dueto problems such as cycle deterioration attributable to interruptions inconductive paths and the like.

In the art described in Patent Document 3, since Si compound particleslocalize on particle surfaces, the Si compound particles dissociate fromthe graphite surface due to volume expansion of the Si compoundparticles accompanying charging and discharging, thereby preventing thisart from reaching the level of practical application due to problemssuch as cycle deterioration attributable to interruptions in conductivepaths and the like.

In the art of Patent Documents 4 to 8, although it is possible togranulate flake graphite or arrange metal particles within particlesduring spheroidization, according to studies conducted by the inventorsof the present invention, even if composite graphite particles areproduced using the methods described in these patent documents, themetal particles present within the composite particles tend toaggregate, and were determined to not satisfy the battery propertiestargeted by the inventors of the present invention. Moreover, theefficiency at which metal particles are contained in the compositeSi-graphite particles is low and leaves room for improvement.

With the foregoing in view, an object of the present invention is toprovide composite graphite particles for a nonaqueous secondary batterynegative electrode without allowing metal particles capable of alloyingwith Li to aggregate within the composite graphite particles, or inother words, allowing the metal particles to be present in a highlydispersed state. As a result thereof, a nonaqueous secondary battery isprovided that has high capacity, high charge-discharge efficiency andsuperior discharge characteristics.

Means for Solving the Problems

As a result of conducting extensive studies to solve the aforementionedproblems, the inventors of the present invention found that a nonaqueoussecondary battery having high capacity, high charge-discharge efficiencyand superior discharge characteristics can be obtained by applying tothe negative electrode material of a nonaqueous secondary batterycomposite graphite particles for a nonaqueous secondary battery negativeelectrode (C) (to also be referred to as “composite graphite particles(C)”) containing graphite (A) and metal particles capable of alloyingwith Li (B) (to also be referred to as “metal particles (B)”), thecomposite graphite particles (C) having a special characteristic to besubsequently described when observed with a scanning electron microscope(SEM).

When the composite graphite particles (C) of the present invention areobserved by SEM, the degree of dispersion of the metal particles (B) inthe composite graphite particles (C) as calculated according to themeasurement method indicated below is 0.78 or more.

Although the detailed mechanism behind the usefulness of theaforementioned composite graphite particles (C) as a negative electrodematerial of a nonaqueous secondary battery is not understood, as aresult of enclosing the metal particles (B) in the composite graphiteparticles (C), the possibility of the metal particles (B) making directcontact with the electrolytic solution decreases in comparison with anegative electrode material of the same capacity, namely containing thesame amount of metal particles (B). Consequently, the irreversible lossof Li ions attributable to the reaction between the metal particles (B)and the nonaqueous electrolytic solution is reduced, or in other words,charge-discharge efficiency is improved.

In addition, since the composite graphite particles (C) of the presentinvention have the metal particles (B) dispersed within compositeparticles, even when compared with commonly known granulated compositegraphite particles having the metal particles (B) enclosed therein orcomposite particles in which a larger amount of the metal particles (B)are added to the outside of graphite particles than to the inside of theparticles, localized expansion and contraction (relaxation) are absorbedby the metal particles (B) present within the particles, therebyresulting in decreased likelihood of the occurrence of collapse of thecomposite graphite particles (C) caused by volume expansion and thesubsequent interruption of conductive paths. As a result, highcharge-discharge efficiency and superior discharge characteristics arethought to be demonstrated.

Namely, the gist of the present invention lies in <1> to <6> indicatedbelow.

<1> Composite graphite particles for a nonaqueous secondary batterynegative electrode comprising graphite and metal particles capable ofalloying with Li, wherein the degree of dispersion of the metalparticles in the composite graphite particles as calculated according tothe measurement method indicated below is 0.78 or more.

(Measurement Method)

When a lattice is drawn in the form of a grid having a length of 2 μmper side (however, length per side A/10 μm in the case the length of thelong axis <20 μm) for each of the images of the scanning electronmicroscope (SEM) of the cross-sections of 10 composite graphiteparticles satisfying the following condition:|0.5×(A+B)−R|≤3(wherein, A represents the length of the long axis (μm), B representsthe length of the short axis (μm), and R represents the mean particlediameter d50 (μm)), the number of squares in the lattice that containcomposite graphite particles are defined as x, and the number of squaresin the lattice containing composite graphite particles that also containmetal particles are defined as y, then the values of y/x for any 5particles are calculated, and the average value thereof is defined asthe degree of dispersion.

<2> The composite graphite particles for a nonaqueous secondary batterynegative electrode described in <1> above, wherein the tapped density ofthe composite graphite particles for a negative electrode is 0.8 g/cm³or more.

<3> The composite graphite particles for a nonaqueous secondary batterynegative electrode described in <1> or <2> above, wherein the metalparticles are contained at 1% by weight to 30% by weight.

<4> The composite graphite particles for a nonaqueous secondary batterynegative electrode described in any one of <1> to <3> above, wherein thespecific surface area as determined by the BET method is 0.1 m²/g to 20m²/g.

<5> An active material for a nonaqueous secondary battery negativeelectrode comprising the composite graphite particles for a nonaqueoussecondary battery negative electrode described in any one of <1> to <4>above, and one or more types of materials selected from the groupconsisting of natural graphite, artificial graphite, carbonaceousmaterial-coated graphite, resin-coated graphite and amorphous carbon.

<6> A nonaqueous secondary battery provided with a positive electrodeand negative electrode, capable of occluding and releasing metal ions,and an electrolytic solution, wherein the negative electrode is providedwith a current collector and a negative electrode active material formedon the current collector, and the negative electrode active materialcontains the active material for a nonaqueous secondary battery negativeelectrode described in any one of <1> to <5> above.

Effects of the Invention

As a result of using the composite graphite particles (C) for anonaqueous secondary battery negative electrode according to the presentinvention as a negative electrode active material of a negativeelectrode for a nonaqueous secondary battery, a nonaqueous secondarybattery can be provided that demonstrates high capacity, highcharge-discharge efficiency and superior discharge characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cross-sectional image of compositegraphite particles (C) indicating one example of a method for measuringthe degree of dispersion of metal particles (B) in the compositegraphite particles (C).

FIG. 2 is a scanning electron microscope (SEM) image of a cross-sectionof composite graphite particles of Example 1.

FIG. 3 is a scanning electron microscope (SEM) image of a cross-sectionof composite graphite particles of Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The following provides a detailed description of the contents of thepresent invention. Furthermore, explanations of constituents of thepresent invention described below are merely examples (representativeexamples) of embodiments of the present invention, and the presentinvention is not limited to these embodiments provided the gist thereofis not exceeded.

The composite graphite particles (C) for a nonaqueous secondary batterynegative electrode of the present invention are composite graphiteparticles (C) for a nonaqueous secondary battery negative electrode thatcontain graphite (A) and metal particles (B) capable of alloying withLi, wherein the degree of dispersion of the metal particles (B) in thecomposite graphite particles (C) when viewing a cross-section of thecomposite graphite particles (C) with a scanning electron microscope ascalculated according a specific measurement method to be subsequentlydescribed is 0.78 or more.

These composite graphite particles (C) are preferably produced by mixingat least the graphite (A) and the metal particles (B) capable ofalloying with Li followed by spheroidizing treatment.

<Graphite (A)>

Although one example of the graphite (A), which is one constituentcomponent of the composite graphite particles (C) of the presentinvention, is indicated below, there are no particular limitations onthe graphite (A), conventionally known graphite and commerciallyavailable products may be used, and the graphite (A) may be fabricatedby any production method.

(Types of Graphite (A))

The graphite (A) can be obtained by carrying out removal of impurities,crushing, screening and classification treatment as necessary on flake,bulk or plate natural graphite or flake, bulk or plate artificialgraphite produced by heating, for example, petroleum coke, coal pitchcoke, coal-based needle coke or mesophase pitch to 2500° C. or higher.

Among these, natural graphite is classified into flake graphite,crystalline (vein) graphite or amorphous graphite (refer to section onGraphite in “Compilation of Particulate Process Technologies”,Industrial Technology Center, 1974) and “Handbook of Carbon, Graphite,Diamond and Fullerenes”, Noyes Publications).

Since vein graphite has the highest degree of graphitization at 100%followed by flake graphite as 99.9%, these types of graphite are usedpreferably.

Natural graphite in the form of flake graphite is produced in countriessuch as Madagascar, China, Brazil, Ukraine and Canada, while veingraphite is mainly produced in Sri Lanka. The main production sites ofamorphous graphite include the Korea peninsula, China and Mexico.

Among these types of natural graphite, since flake graphite and veingraphite offer the advantages of high degrees of graphitization and lowlevels of impurities, they can be used preferably in the presentinvention.

Examples of visual techniques for confirming that graphite is flakegraphite include observing particle surfaces with a scanning electronmicroscope, and after having embedding particles in resin to preparethin resin sections and then cutting out particle cross-sections, orafter having prepared a coated film cross-section from a coated filmcomposed of particles using a cross-section polisher and cutting outparticle cross-sections, the particle cross-sections are observed with ascanning electron microscope.

Flake graphite and vein graphite consist of natural graphite, for whichpurity has been enhanced so as to demonstrate crystals havingcrystallinity completely similar to that of graphite, and artificiallyformed graphite, and is preferably natural graphite from the viewpointof being industrially inexpensive.

(Physical Properties of Graphite (A))

Physical properties of the graphite (A) in the present invention areindicated below. Furthermore, although there are no particularlimitations on measurement methods used in the present invention, theycomply with the measurement methods described in the examples unlessthere are special circumstances.

(1) Volume Mean Particle Diameter (d50) of Graphite (A)

The volume mean particle diameter (d50) of the graphite (A) prior tocompounding with the metal particles (B) (to also be referred to as“d50” in the present invention) is normally 1 μm to 50 μm, preferably 2μm to 40 μm and more preferably 5 μm to 30 μm. If the volume meanparticle diameter (d50) is within these ranges, composite graphiteparticles (C) can be produced that are embedded with the metal particles(B). In addition, if the volume mean particle diameter (d50) of thegraphite (A) is excessively large, the particle diameter of thecomposite graphite particles (C) embedded with the metal particles (B)becomes large, thereby resulting in the formation of streaks and surfaceirregularities caused by large particles in a step for coating anelectrode material mixed with the composite graphite particles (C) inthe form of a slurry by adding a binder, water or organic solvent. Ifthe volume mean particle diameter is excessively small, the specificsurface area of the composite graphite particles (C) increases,resulting in the risk of an increase in side reactions with theelectrolytic solution.

Here, volume mean particle diameter (d50) refers to the volume-basedmedian diameter as measured by laser diffraction scattering particlesize distribution measurement.

(2) Mean Aspect Ratio of Graphite (A)

Mean aspect ratio, which is the ratio of the length of the long axis tothe length of the short axis of the graphite (A) prior to compoundingwith the metal particles (B), is normally 1 to 50, preferably 3 to 30and more preferably 5 to 20. If mean aspect ratio is within theseranges, the metal particles (B) within the composite graphite particles(C) can be arranged with favorable dispersibility.

Furthermore, in the present invention, aspect ratio refers to the ratioof the length of the long axis to the length of the short axis ofparticles, and since the minimum value thereof is 1, the lower limit ofaspect ratio is normally 1. Aspect ratio is measured by capturingelectron micrographs of target particles, selecting 20 particles withina randomly selected region, defining the longest axis of each particleas α (μm) and the shortest axis as β (μm), determining the ratio α/β,and taking the mean value of α/β of the 20 particles to be the aspectratio.

(3) Tapped Density of Graphite (A)

The tapped density of graphite (A) prior to compounding with the metalparticles (B) is normally 0.1 g/cm³ to 1.0 g/cm³, preferably 0.13 g/cm³to 0.8 g/cm³ and more preferably 0.15 g/cm³ to 0.6 g/cm³. If the tappeddensity of the graphite (A) is within the aforementioned ranges, themetal particles (B) can be dispersed at a high degree of dispersion inthe composite graphite particles (C).

In the present invention, tapped density is defined as the densitydetermined by dropping a raw material carbon material through a sievehaving an opening size of 300 μm into a cylindrical tapping cell havinga diameter of 1.6 cm and volume capacity of 20 cm³ using a powderdensity tester and completely filling the cell, followed by tapping 1000times at a stroke length of 10 mm and determining density from thevolume and sample weight at that time.

(4) Specific Surface Area of Graphite (A) Determined According to BETMethod

Specific surface area of the graphite (A) prior to compounding with themetal particles (B) as determined according to the BET method isnormally 1 m²/g to 40 m²/g, preferably 2 m²/g to 35 m²/g, and morepreferably 3 m²/g to 30 m²/g. Specific surface area of the graphite (A)according to the BET method is reflected in the specific surface rea ofthe composite graphite particles (C). Consequently, by making thespecific surface area of the graphite (A) to be 40 m²/g or less,decreases in battery capacity caused by irreversible increases incapacity when using the composite graphite particles (C) in an activematerial for a nonaqueous secondary battery negative electrode can beprevented.

In the present invention, specific surface area is measured according tothe BET multipoint method using nitrogen gas adsorption.

(5) Interplanar Spacing of 002 Plane (d002) and Lc of Graphite (A)

Interplanar space of the 002 plane (d002) of the graphite (A) asdetermined by wide-angle X-ray diffraction is normally 0.337 nm or less.On the other hand, since the theoretical value of the interplanarspacing of the 002 plane of graphite is 0.335 nm, the interplanarspacing of the 002 plane of graphite is normally 0.335 nm or more.

In addition, crystallite size (Lc) of the graphite (A) in the directionof the c axis as determined by wide-angle X-ray diffraction is 90 nm ormore and preferably 95 nm or more.

If the interplanar spacing of the 002 plane (d002) is 0.337 nm or less,the graphite (A) demonstrates high crystallinity, allowing the obtainingof high-capacity composite graphite particles (C). In addition, in thecase Lc is 90 nm or more as well, the graphite (A) demonstrates highcrystallinity, allowing the obtaining of a high-capacity negativeelectrode material that uses the composite graphite particles (C)containing the graphite (A).

(6) True Density of Graphite (A)

True density of the graphite (A) prior to compounding with the metalparticles (B) is normally 2.1 g/cm³ or more, preferably 2.15 g/cm³ ormore and more preferably 2.2 g/cm³ or more. Graphite that demonstrateshigh crystallinity having a true density of 2.1 g/cm³ or more allows theobtaining of high-capacity composite graphite particles (C) having lowirreversible capacity.

(7) Lengths of Long and Short Axes of Particles of Graphite (A)

The length of the long axis of the graphite (A) prior to compoundingwith the metal particles (B) is preferably 40 μm or less, morepreferably 35 μm or less and even more preferably 30 μm or less, andpreferably 3 μm or more, more preferably 5 μm or more and even morepreferably 8 μm or more.

In addition, the length of the short axis of the graphite (A) isnormally 0.9 μm to 5 μm and preferably 1.5 μm to 4 μm. If the lengths ofthe long and short axes of the graphite (A) are within theaforementioned ranges, the degree of dispersion of the metal particles(B) in the composite graphite particles (C) improves.

(8) Oil Absorption of Graphite (A)

The amount of oil absorbed by the graphite (A) prior to compounding withthe metal particles (B) when using dibutyl phthalate (DBP) is preferably50 ml/100 g or more and more preferably 80 ml/100 g or more, andpreferably 200 ml/100 g or less and more preferably 200 ml/100 g orless. Although there are no problems when oil absorption is excessivelylow, if oil absorption is excessively high, when the graphite (A) andmetal particles (B) are mixed in a slurry and subjected to shearingforce, a larger amount of solvent is required thereby resulting in therisk of poor productivity.

Furthermore, characteristics of the graphite (A) after having beencompounded with the metal particles (B) are preferably within the rangeof the characteristics of the graphite (A) prior to compounding with themetal particles (B).

<Metal Particles (B) Capable of Alloying with Li>

In the composite graphite particles (C) of the present invention, themetal particles (B) capable of alloying with Li are mainly embeddedwithin the composite graphite particles (C) although a portion thereofmay be present on the surface of the composite graphite particles (C).

(Types of Metal Particles (B) Capable of Alloying with Li)

Although conventionally known metal particles can be used for the metalparticles (B) capable of alloying with Li, a metal or compound thereofselected from the group consisting of Fe, Co, Sb, Bi, Pb, Ni, Ag, Si,Sn, Al, Zr, Cr, V, Mn, Nb, Mo, Cu, Zn, Ge, As, In, Ti and W ispreferable from the viewpoints of capacity and cycle life. In addition,an alloy composed of two or more types of metals may be used, or themetal particles may be alloy particles formed from two or more types ofmetal elements. Among these, a metal or compound thereof selected fromthe group consisting of Si, Sn, As, Sb, Al, Zn and W is preferable.

In addition, although the metal particles (B) can be in the form ofsingle crystals, polycrystals or in amorphous form regardless of thecrystalline state thereof, they are preferably in the form ofpolycrystals from the viewpoints of facilitating reduced particlediameter and being able to anticipate higher capacity and high C-ratecharacteristics.

Examples of metal compounds include metal oxides, metal nitrides andmetal carbides. In addition, metal compounds composed of two or moretypes of metals may also be used.

Si and/or Si compounds are preferable form the metal particles (B) fromthe viewpoint of high capacity. In the present description, Si and/or Sicompounds are collectively referred to as Si compounds.

Specific examples of Si compounds include those represented by thegeneral formulas Si, SiOx, SiNx and SiCx. In addition, Si compounds maybe doped from a portion of the surface to the inside thereof by formingbonds between Si and elements such as C, N, P or F. Although the generalformula SiOx is obtained by using raw materials consisting of silicondioxide (SiO₂) and metal silicon (Si), the value of x is normally suchthat 0<x<2, preferably 0.05 to 1.8, more preferably 0.1 to 1.6 and evenmore preferably 0.15 to 1.4. If the value of x is within these ranges,irreversible capacity caused by bonding between Li and oxygen can bereduced while simultaneously increasing capacity.

Bonding between the metal particles (B) and the aforementioned atoms canbe analyzed by methods such as X-ray photoelectron spectroscopy (XPS),infrared spectroscopy (IR) or X-ray absorption fine structure analysis(XAFS).

The aforementioned sputtering treatment or a method consisting of mixingin a compound having the aforementioned atoms during mechanical energytreatment or high temperature treatment can be used to form bondsbetween the metal particles (B) and the aforementioned atoms. Inaddition, the metal particles (B) containing the aforementioned atomscan also be used as a raw material.

Impurities other than those previously described may be present withinthe Si compound or on the surface thereof.

(Physical Properties of Metal Particles (B) Capable of Alloying with Li)

Although there are no particular limitations on the metal particles (B)capable of alloying with Li in the present invention provided they areable to alloy with Li, the metal particles (B) preferably demonstratethe physical properties indicated below. Furthermore, although there areno particular limitations on the measurement methods used in the presentinvention, they comply with the measurement methods described in theexamples unless there are special circumstances.

(1) Volume Mean Particle Diameter (d50) of Metal Particles (B)

The volume mean particle diameter (d50) of the metal particles (B) inthe composite graphite particles (C) is normally 0.005 μm or more,preferably 0.01 μm or more, more preferably 0.02 μm or more and morepreferably 0.03 μm or more, and normally 10 μm or less, preferably 9 μmor less and more preferably 8 μm or less. If the volume mean particlediameter (d50) is within these ranges, volume expansion accompanyingcharging and discharging is reduced and favorable cyclingcharacteristics can be obtained while maintaining charge-dischargecapacity.

Si and SiOx have a large theoretical capacity in comparison withgraphite, and amorphous Si or nano-sized Si crystals facilitate themigration of alkaline ions such as lithium ions, thereby allowing theobtaining of high capacity. Polycrystalline Si is preferably used toobtain the aforementioned ranges from the viewpoints of facilitatingreductions in particle diameter and demonstrating superior cyclingcharacteristics and C-rate characteristics.

(2) Specific Surface Area of Metal Particles (B) Determined According toBET Method

Specific surface area of metal particles (B) in the composite graphiteparticles (C) as determined according to the BET method is normally 0.5m²/g to 120 m²/g and preferably 1 m²/g to 100 m²/g. If specific surfacearea of the metal particles capable of alloying with Li as determinedaccording to the BET method is within the aforementioned ranges,charge-discharge efficiency and discharge capacity of the resultingbattery are high, migration of Li ions is rapid during high-speedcharging and discharging, and rate characteristics are superior, therebymaking this preferable.

(3) Oxygen Content of Metal Particles (B)

Although there are no particular limitations thereon, the oxygen contentof the metal particles (B) in the composite graphite particles (C) isnormally 0.01% by weight to 50% by weight and preferably 0.05% by weightto 30% by weight. Although the distribution of oxygen within theparticles may be near the surface, within the particles or uniformlywithin the particles, it is preferably present near the surface inparticular. If the oxygen content of the metal particles is within theaforementioned ranges, in addition to demonstrating superior initialefficiency, volume expansion accompanying charging and discharging isinhibited and cycling characteristics are superior due to the strongbonding between Si and O, thereby making this preferable.

(4) Purity of Metal Particles (B)

Although there are no particular limitations thereon, purity of themetal particles (B) in the composite graphite particles (C) ispreferably 70% or more, more preferably 80% or more and even morepreferably 90% or more. If purity is within the aforementioned ranges,capacity can be obtained that is close to the theoretical capacity ofthe metal particles (B). In addition, although there are no particularlimitations on the composition of impurities present in the metalparticles (B), the content of substances that cause side reactions in anonaqueous secondary battery is preferably low.

(5) Crystallite Size of Metal Particles (B)

Although there are no particular limitations on crystallite size in thecase the metal particles (B) within the composite graphite particles (C)are crystals, crystallite size of the (111) plane as calculated by X-raydiffraction (XRD) is normally 0.05 nm or more and preferably 1 nm ormore, and normally 100 nm or less and preferably 50 nm or less. If thecrystallite size of the metal particles is within the aforementionedranges, the reaction between Si and Li ions proceeds rapidly, there islittle susceptibility to the occurrence of dissociation of metalparticles due to the formation of cracks since Li concentration in themetal particles is uniform, cycling characteristics are superior, andboth input-output and rate characteristics are superior, thereby makingthis preferable.

Furthermore, characteristics of the metal particles (B) prior tocompounding with the graphite (A) are preferably within the ranges ofthe characteristics of the metal particles (B) within the compositegraphite particles (C) as described above.

(Production Method of Metal Particles (B) Capable of Alloying With Li)

(1) Previous Production Methods

The bottom-up approach is known to be one method for currently producingfine particles of the metal particles (B). This bottom-up approachconsists of temporarily decomposing a material to the atomic ormolecular level by reacting or evaporating followed by re-solidifying.Examples of vapor phase methods include plasma-enhanced vapor depositionthat utilizes a chemical reaction (see, for example, Japanese UnexaminedPatent Publication No. H6-279015) or arc melting consisting ofphysically evaporating a material (see, for example, Japanese UnexaminedPatent Publication No. 2005-097654). Moreover, known examples of liquidphase methods include methods capable of defining a dispersed state in aliquid phase such as the co-precipitation method, reverse micelle methodor hot soap method (see, for example, Japanese Unexamined PatentPublication No. 2003-515459). In addition, fine particles can also besynthesized using the top-down approach in which large particles arecrushed into fine particles (see, for example, Japanese UnexaminedPatent Publication No. H7-88391). An example of the aforementionedtop-down approach is a method using a wet bead mill that carries outcrushing using beads for the media (see, for example, JapaneseUnexamined Patent Publication No. 2009-235263).

Although either the bottom-up approach or top-down approach can be usedto produce the fine particles (B), producing the fine particles (B)using the top-down approach is preferable in the case of consideringindustrial productivity.

(2) Production Method of Metal Particles (B) Using Top-Down Approach

The metal particles (B) capable of alloying with Li are preferably usedafter selecting and combining crushers used according to the size of theraw materials and the target particle diameter. For example, a mediamill (ball mill), vibration mill, pulverizer or jet mill and the like isused for the dry crusher if the target diameter is about 5 μm, and ispreferably used together with coarse crusheres as necessary according tothe raw material size. In the case the submicron range or smaller is thetarget particle diameter, a wet bead mill is preferable. Althoughexamples of methods used by the crusher to separate the beads and mediainclude gap separation, screen separation and centrifugal separation,any of these methods can be used provided they enable beads to beseparated that are required for achieving the target particle diameter.In addition, since there are cases in which crushing is not completed byonly passing particles through the crusher once, a system that allowscirculation of solvent is preferable. Although there are no particularlimitations thereon, examples include the Starmill LMZ Nanogetter HFMmanufactured by Ashizawa Finetech Ltd., Pico Mill PCM manufactured byAsada Iron Works Co., Ltd., UVM Alpha Mill AM manufactured by AimexCorp., and Ultra Apex Mill UAM manufactured by Kotobuki Kogyo Co., Ltd.

(3) Slurry Composition

Raw Material of Metal Particles (B) Capable of Alloying with Li

Commercially available metal particles may be used for the metalparticles (B) provided they satisfy the characteristics of the presentinvention.

In addition, although there are no particular limitations on theproduction method, metal particles produced according to the methoddescribed in Japanese Patent No. 3952118, for example, can be used forthe metal particles (B). In the case of producing SiOx, for example,after mixing silicon dioxide powder and silicon metal powder at aspecific ratio and filling this mixture into a reactor, SiOx gas isgenerated by heating to 1000° C. or higher and holding at thattemperature either at normal pressure or after reducing pressure to aspecific pressure followed by cooling and precipitating the gas toobtain particles represented by the general formula SiOx (sputteringtreatment). The precipitate can be used in the form of particles byapplying mechanical energy treatment.

Although conventionally known raw materials can be used for the rawmaterial of the metal particles (B), from the viewpoints of capacity andcycle life, a metal or compound thereof selected from the groupconsisting of Fe, Co, Sb, Bi, Pb, Ni, Ag, Si, Sn, Al, Zr, Cr, V, Mn, Nb,Mo, Cu, Zn, Ge, As, In, Ti and W is preferable. In addition, an alloycomposed of two or more types of metals may be used, and the metalparticles may be alloy particles formed from two or more types of metalelements. Among these, a metal or compound thereof selected from thegroup consisting of Si, Sn, As, Sb, Al, Zn and W is preferable.Furthermore, the raw material of the metal particles can be used in theform of single crystals, polycrystals or amorphous form regardless ofthe crystalline state thereof. Although there are no particularlimitations thereon, in the case of an Si compound, for example, crushedingots having purity of any of metal grade (3N), solar cell grade (6N)or semiconductor grade (9N) may be used, and either single crystal orpolycrystalline ingots may be used (see, for example, JapaneseUnexamined Patent Publication No. 2012-206923).

Dispersion Solvent

Although there are no particular limitations on the dispersion solventused, that having favorable wettability with the raw material of themetal particles (B) is preferable from the viewpoint of improvingdispersibility of the metal particles (B) in the composite graphiteparticles (C). In addition, a slurry used with a wet bead mill hasfavorable dispersibility, and that having low viscosity is preferablefrom the viewpoint of productivity. Moreover, a trace amount of adispersant (surfactant) may be added to further increase wettability anddispersibility. A dispersant having no or extremely little reactivitywith the metal particles (B) is preferably suitably selected for thedispersant.

The use of a nonpolar solvent or aprotic polar solvent is preferable forinhibiting reactions between solvent and newly-formed surfaces of themetal particles (B) following crushing, and that having an aromatic ringis particularly preferable. Although there are no particular limitationson the type of nonpolar compound having an aromatic ring, that whichexhibits extremely little reactivity with the raw material of the metalparticles (B) is more preferable. Examples thereof include liquidaromatic compounds such as benzene, toluene, xylene, cumene ormethylnaphthalene, alicyclic hydrocarbons such as cyclohexane,methylcyclohexane, methylcyclohexene or bicyclohexyl, and residual oilsof petrochemicals and coalchemicals in the manner of light oil or heavyoil. Among the aforementioned solvents, xylene is preferable from theviewpoint of crushing efficiency due to its low solvent viscosity andease of drying.

In addition, solvents that dissolve not only water but also organicsolvents are preferable for the aprotic polar solvent, examples of whichinclude glycol-based solvents not having a hydroxyl group such asN-methyl-2-pyrrolidone (NMP), γ-butyrolactone (GBL),N,N-dimethylformamide (DMF) or propylene glycol monoethyl ether acetate(PGMEA). N,N-dimethylformamide (DMF) is preferable from the viewpoint offavorable dispersibility with Si compounds, its low boiling point andits low viscosity.

In the case of crushing with a protic polar solvent, it is preferablyused by inhibiting decomposition gas.

The proportion of dispersion medium in the metal particles (B) anddispersion medium is normally 50% by weight or more and preferably 60%by weight or more and normally 90% by weight or less and preferably 80%by weight or less.

If the proportion of dispersion medium is excessively high, costs tendto increase when considering drying, and container size also becomeslarger resulting in increased susceptibility to unevenness. Conversely,if the mixed proportion of the dispersion medium is excessively low,slurry viscosity increases due to a decrease in average distance betweenparticles and slurry dispersibility tends to become poor.

Types of Dispersants

A dispersant may be added to the aforementioned dispersion medium whenproducing the metal particles (B). The dispersant is only required to beable to dissolve in each dispersion medium, and may be a low molecularweight surfactant that improves wettability. Examples thereof includeNoigen TDS30, Noigen EN, Noigen ET65, Noigen ET115, Noigen ES-149D,DKS-NL-DASH400, DKS-NL-DASH408, Amiradine C1802, Dianol CDE, PlysurfA208B, Plysurf A208F, Catiogen ESL-9, Noigen TDS100, Noigen XL40, NoigenXL80, Noigen TDX50, Noigen LF60, Noigen EA87, Noigen EA167, Sorgen 30V,Noigen ET69, Noigen ET149, Noigen ES99D, DKSNL15, DKSNL50, Eban 410(Dai-ichi Kogyo Seiyaku Co., Ltd.), Nopcosperse 092, SN Dispersant 9228,SN Sperse 70 (San Nopco Ltd.), PW36, DA375, KS806, KS873, 1831, 1850,DA1401, 1860 (Kusumoto Chemicals, Ltd.), Arquad 22-80, Armin 8D, ArminCD, Armin M20, Duomin CD, Duomin T, Esomin T/12, Erimin 0/12 and ArminOD. In addition, a high molecular weight dispersant is more preferablein which the target system is able to realize a dispersion system due tosteric hindrance effects as particle diameter decreases. Examplesthereof include Ajisper PA111, Ajisper PB821, Ajisper PB822, AjisperPB881, Ajisper PN411 (Ajinomoto Co., Inc.), Anti-Terra U100,Byk-LPN6919, Disperbyk-102, Disperbyk-103, Disperbyk-111, Dispebyk-118,Disperbyk-167, Disperbyk-180, Disperbyk-2000, Disperbyk-2001,Disperbyk-2009, Dipserbyk-2013, Disperbyk-2022, Disperbyk-2050,Disperbyk-2152, Disperbyk-2155, Disperbyk-2164, Disperplast-1142,Disperplast-1148, Disperplast-1150 (Byk Chemie GmbH), Hinoact KF-100,Hinoact KF-1500, Hinoact T-6000, Hinoact T-8000, Hinoact T-8000E,Hinoact T-9100 (Kawaken Fine Chemicals Co., Ltd.), DA-703-50, DA375,DA1200, SPD200, SPD201, SPD201, DA325 and DN900 (Kusumoto Chemicals,Ltd.).

<Other Materials>

The composite graphite particles (C) for a nonaqueous secondary batterynegative electrode of the present invention may contain materials otherthan the graphite (A) and the metal particles (B).

(Carbon Fine Particles)

The composite graphite particles (C) of the present invention may alsocontain carbon fine particles for improving electrical conductivity.

Volume Mean Particle Diameter (d50)

The volume mean particle diameter of the carbon fine particles isnormally 0.01 μm to 10 μm, preferably 0.05 μm or more, more preferably0.07 μm or more and even more preferably 0.1 μm or more, and preferably8 μm or less, more preferably 5 μm or less and even more preferably 1 μmor less.

In the case the carbon fine particles have a secondary structureresulting from the congregation and aggregation of primary particles,although there are no particular limitations on the type or otherproperties thereof provided the primary particle diameter is 3 nm to 500nm, the primary particle diameter is preferably 3 nm or more, morepreferably 15 nm or more, even more preferably 30 nm or more andparticularly preferably 40 nm or more, and preferably 500 nm or less,more preferably 200 nm or less, even more preferably 100 nm or less andparticularly preferably 70 nm or less. The primary particle diameter ofcarbon fine particles can be measured by observing with an electronmicroscope such as an SEM or by measuring with a laser diffractionparticle size distribution analyzer.

Types of Carbon Fine Particles

There are no particular limitations on the shape of the carbon fineparticles, and may have a granular shape, spherical shape, linear shape,needle shape, fibrous shape, plate-like shape or flake-like shape.

More specifically, although there are no particular limitations on thecarbon fine particles, examples thereof includes substances having ananostructure such as coal fine powder, vapor phase carbon powder,carbon black, Ketjen black, fullerene, carbon nanofibers, carbonnanotubes or carbon nanowalls. Carbon black is particularly preferableamong them. The use of carbon black improves input-outputcharacteristics at low temperatures while also simultaneously offeringthe advantages of being inexpensive and readily available.

(Carbon Precursor)

In addition, the composite graphite particles (C) of the presentinvention may also be mixed with a carbon precursor for inhibitingreactions between the metal particles (B) and nonaqueous electrolyticsolution.

Reactions between the metal particles (B) and nonaqueous electrolyticsolution can be inhibited as a result of the carbon precursor coveringthe periphery of the metal particles (B).

Mixing with the carbon precursor may be carried out prior to firing thecomposite particles or may be carried out by dissolving in a crushedslurry containing the metal particles (B) provided it is solubletherein.

Types of Carbon Precursors

Carbon materials described in the following (α) and/or (β) arepreferably used for the aforementioned carbon precursor.

(α) Carbonizable organic matter selected from the group consisting ofcoal-based heavy oil, direct flow heavy oil, cracked petroleum heavyoil, aromatic hydrocarbons, N ring compounds, S ring compounds,polyphenylene, synthetic organic polymers, natural polymers,thermoplastic resins and thermosetting resins

(β) Product of dissolving carbonizable organic matter in low molecularweight organic solvents

Coal tar pitch ranging from soft pitch to hard pitch or dry distillationliquefied oil is preferable for the aforementioned coal-based heavy oil.Atmospheric residue or vacuum residue is preferable for theaforementioned direct flow heavy oil. Ethylene tar produced as aby-product during cracking of crude oil, naphtha and the like ispreferable for the aforementioned cracked petroleum heavy oil.Acenaphthylene, decacyclene, anthracene and phenanthrene are preferablefor the aforementioned aromatic hydrocarbons. Phenazine and acridine arepreferable for the aforementioned N ring compounds. Thiophene andbithiophene are preferable for the aforementioned S ring compounds.Biphenyl or terphenyl is preferable for the aforementionedpolyphenylene. Polyvinyl chloride, polyvinyl alcohol, polyvinyl butyral,insoluble treatment products thereof, nitrogen-containing polymers suchas polyacrylonitrile, polypyrrole, polyallylamine, polyvinylamine,polyethyleneimine, urethane resin and urea resin, polythiophene,polystyrene and polymethacrylic acid are preferable for theaforementioned synthetic organic polymers. Polysaccharides such ascellulose, lignin, mannan, polygalacturonic acid, chitosan or saccharoseare preferable for the aforementioned natural polymers. Polyphenylenesulfide and polyphenylene oxide are preferable for the aforementionedthermoplastic resins. Furfuryl alcohol resins, phenol-formaldehyderesins and imide resins are preferable for the aforementionedthermosetting resins.

In addition, the carbonizable organic matter may also be a carbide of asolution obtained by dissolving in a low molecular weight organicsolvent such as benzene, toluene, xylene, quinoline or n-hexane. Onetype of this carbonizable organic matter may be used alone or two ormore types may be used in an arbitrary combination.

X-Ray Parameters of Carbon Materials Obtained by Firing Carbon Precursor

The d value of interplanar spacing of the (002) plane as determined bywide-angle X-ray diffraction of a powder of a carbon material obtainedby firing the carbon precursor is normally 0.340 nm or more andpreferably 0.342 nm or more. In addition, the d value is normally lessthan 0.380 nm, preferably 0.370 nm or less and more preferably 0.360 nmor less. An excessively large d002 value results in low crystallinityand cycling characteristics tend to decrease, while if the d002 value isexcessively small, it is difficult to obtain the effect of compoundingthe carbon material.

In addition, the crystallite size (Lc(002)) of a carbon material asdetermined by X-ray diffraction according to the Gakushin method of apowder of a carbon material obtained by firing the carbon precursor isnormally 5 nm or more, preferably 10 nm or more and more preferably 20nm or more. In addition, crystallite size (Lc(200)) is normally 300 nmor less, preferably 200 nm or less and more preferably 100 nm or less.If crystallite size is excessively large, cycling characteristics tendto decrease, while if crystallite size is excessively small,charge-discharge reactivity decreases, resulting the risk of increasesin generated gas during high-temperature storage and a decrease inlarge-current charge-discharge characteristics.

(Resin Serving as Void Forming Material)

In addition, the composite graphite particles (C) of the presentinvention may also be mixed with a resin serving as a void formingmaterial in order to alleviate destruction of the composite graphiteparticles (C) caused by expansion and contraction of the metal particles(B). Furthermore, although the resin serving as a void forming materialas referred to in the present description is included in some of theaforementioned carbon precursors, coal-based heavy oil, direct flowheavy oil and cracked petroleum heavy oil are not included.

Molecular Weight

Although there are no particular limitations thereon, the weight averagemolecular weight of the resin serving as a void forming material isnormally 500 or more, preferably 1000 or more, more preferably 1500 ormore, even more preferably 2000 or more and particularly preferably 2500or more. On the other hand, the weight average molecular weight isnormally 1,000,000 or less, preferably 500,000 or less, more preferably300,000 or less, even more preferably 100,000 or less, particularlypreferably 50,000 or less and most preferably 10,000 or less. In thecase the molecular weight is excessively low, specific surface areaincreases resulting in a tendency for charge-discharge efficiency todecrease when contained in particles, while in the case the molecularweight is excessively high, viscosity increases, which tends to makeuniform mixing and dispersion difficult.

Firing Yield

The firing yield of the resin serving as a void forming material isnormally 0.1% or more, preferably 1% or more, more preferably 5% or moreand even more preferably 10% or more, while normally less than 20%,preferably 18% or less, more preferably 16% or less and even morepreferably 14% or less. In the case the firing yield is excessivelyhigh, voids are not formed and the alleviating action accompanyingexpansion and contraction of the metal particles (B) tends to decrease.

Decomposition Temperature

The decomposition temperature of the void forming material is normally30° C. or higher, preferably 50° C. or higher, more preferably 100° C.or higher and even more preferably 150° C. or higher, while normally500° C. or lower, preferably 400° C. or lower, more preferably 300° C.or lower and even more preferably 200° C. or lower. In the case thedecomposition temperature is excessively low, there is the risk of thevoid forming material easily decomposing, while in the case thedecomposition temperature is excessively high, it becomes difficult forthe void forming material to dissolve in solvent, thereby resulting inthe risk of making it difficult to disperse the void forming materialuniformly.

Types of Resins

Although there are no particular limitations thereon, examples of resinsthat can be used for the void forming material include polyvinylalcohol, polyethylene glycol, polycarbosilane, polyacrylic acid andcellulose-based polymers, and polyvinyl alcohol and polyethylene glycolcan be used particularly preferably from the viewpoint of low remainingcarbon on firing and having a comparatively low decompositiontemperature.

(Other Resins)

In addition, the metal particles (B) may also contain a resin so as tocover the surface of the metal particles (B). Silane couplingagent-derived resins that are able to bind to silanol groups (see, forexample, Japanese Unexamined Patent Publication No. 2006-196338) andresins having functional groups demonstrating high affinity for themetal particles (B) are preferable.

<Composite Graphite Particles (C) for Nonaqueous Secondary BatteryNegative Electrode>

The composite graphite particles (C) of the present invention containgraphite (A) and metal particles (B) capable of alloying with Li, andwhen cross-sections of the composite graphite particles (C) are observedwith a scanning electron microscope (SEM), the degree of dispersion ofthe aforementioned metal particles (B) in the composite graphiteparticles (C) as calculated according to the measurement method to besubsequently described is 0.78 or more.

Degree of Dispersion of Metal Particles (B) Within Composite GraphiteParticles (C)

The degree of dispersion of the metal particles (B) in the compositegraphite particles (C) as measured according to the method used tomeasure the composite graphite particles (C) of the present inventionindicated below is 0.78 or more, preferably 0.8 or more, more preferably0.85 or more, even more preferably 0.87 or more, particularly preferably0.89 or more and most preferably 0.92 or more. In addition, the maximumdegree of dispersion is 1. A higher value within the aforementionedranges indicates that the metal particles (B) are uniformly dispersed inthe composite graphite particles (C), and when used to form a negativeelectrode, particle destruction caused by local expansion within theparticles can be prevented, reductions in charge-discharge efficiency ineach cycle can be inhibited, and rate characteristics tend to besuperior.

The degree of dispersion of the metal particles (B) in the compositegraphite particles (C) of the present invention is calculated in themanner indicated below. It is necessary to observed particlecross-sections of the composite graphite particles (C) in order tocalculate degree of dispersion. Although there are no particularlimitations on the method used to observe particle cross-sections,particle cross-sections can be observed by an observation methodconsisting of preparing a polar plate of the composite graphiteparticles (C), a coated film of the composite graphite particles (C), orresin thin sections by embedding the composite graphite particles (C) inresin and the like, followed by cutting the prepared specimens with afocused ion beam (FIB) or by ion milling, cutting out particlecross-sections and then observing the particle cross-sections with ascanning electron microscope (SEM).

The accelerating voltage when observing cross-sections of primaryparticles of the composite graphite particles (C) with a scanningelectron microscope (SEM) is normally preferably 1 kV or more, morepreferably 2 kV or more and even more preferably 3 kV or more, andnormally 10 kV or less, more preferably 8 kV or less and even morepreferably 5 kV or less. If accelerating voltage is within these ranges,it becomes easy to distinguish between graphite particles and Sicompounds due to differences in reflected secondary electron imagespresent in the resulting SEM images. In addition, the imagingmagnification factor is normally 500× or more, more preferably 1000× ormore and even more preferably 2000× or more, and normally 10000× orless. If the magnification factor is within the aforementioned ranges,it becomes possible to acquire an image of an entire primary particle ofthe composite graphite particles (C). Resolution is 200 dpi (ppi) ormore and preferably 256 dpi (ppi) or more. In addition, the images arepreferably evaluated at 800 pixels or more. Although elements of thegraphite (A) and metal particles (B) may be identified byenergy-dispersive X-ray spectroscopy (EDX) and wavelength-dispersiveX-ray spectroscopy (WDX) while observing images, since, for example, theelectron conductivity of Si compounds is normally not favorable, theytend to appear white particularly in the case of reflected electronimages. Consequently, they can be easily distinguished from graphite (A)and amorphous carbon. In the case graphite (A) and metal particles (B)are easily distinguished, particles having a white reflection inreflected electron images and the like are defined as metal particles(B).

Furthermore, since there is the risk of the boundaries between metalparticles (B) and graphite (A) becoming ambiguous in cases of lowresolution during EDX (EDS) or WDX, mapping is preferably carried out ata high resolution roughly equal to that during SEM when carrying outanalyses by mapping images.

Degree of Dispersion Measurement Method and Conditions

The method used to measure degree of dispersion is defined as satisfyingconditions 1 to 4 indicated below.

(Condition 1)

100 or more composite graphite particles (C) are randomly selected in anacquired image (and preferably, a polar plate). At this time, theparticles are demarcated in contour units of the composite graphiteparticles (C). Although there are no particular limitations on themethod used to demarcate the particles, any image processing softwaremay be used provided background and the distinction between light anddark are well-defined, although the boundaries may be defined manuallyin cases in which distinction is difficult. Although the particles maybe approximated with polygons, they are preferably minimallyapproximated with hectogons or better to prevent the particles frombecoming coarse. The center of gravity (centroid) is then defined with asingle, arbitrary extracted composite graphite particle (C). First, theparticle demarcated from this boundary is approximated with squares.Although the size of the squares is not particularly specified, itpreferably corresponds to an actual dimension of 5 nm or less.Two-dimensional coordinates are then determined on the image. Thecoordinates at the center of the squares is defined. The squares arethen numbered from 1 to N assuming the squares to have the same weight.The coordinates of the center of gravity of the composite particle arethen determined from the following equation 1.

$\begin{matrix}\left\lbrack {{Equation}1} \right\rbrack & \end{matrix}$ $\begin{matrix}{\overset{\rightarrow}{r_{G}} = \frac{\overset{N}{\sum\limits_{1}}\overset{\rightarrow}{r_{i}}}{N}} & (1)\end{matrix}$

Here, r_(i) represents the coordinates of the ith square and r_(G)represents the coordinates of the center of gravity. The procedure fordetermining the center of gravity may be carried out with any imagingsoftware, and the demarcated grid may be determined with the followingequation provided each center of gravity can be defined with anarbitrary figure.

$\begin{matrix}\left\lbrack {{Equation}2} \right\rbrack & \end{matrix}$ $\begin{matrix}{\overset{\rightarrow}{r_{G}} = \frac{\overset{N}{\sum\limits_{1}}{A_{i}\overset{\rightarrow}{r_{i}}}}{\overset{N}{\sum\limits_{1}}A_{i}}} & (2)\end{matrix}$

Here, A_(i) represents the area of the ith figure and r_(i) representsthe coordinates of the center of gravity (centroid) of the ith figure.

Next, the longest line segment among the arbitrary line segmentsdemarcated by the aforementioned established boundary according to thedetermined center of gravity is defined as the long axis. In addition,the line segment that is perpendicular to the long axis among thearbitrary line segments demarcated by the aforementioned establishedboundary according to the center of gravity is defined as the shortaxis.

(Condition 2)

The 100 or more composite graphite particles (C) randomly selectedaccording to Condition 1 are subjected to processing in the mannerdescribed above to determine the center of gravity, long axis and shortaxis, and 10 particles that satisfy the following condition are randomlyselected.|0.5×(A+B)−R|≤3Here, A represents the length of the long axis (μm), B represents thelength of the short axis (μm), and R represents the mean particlediameter d50 (μm).

(Condition 3)

A grid-like lattice is drawn for the 10 composite graphite particles (C)selected according to Condition 2. The lattice is placed so as to beparallel with the long axis of the particles (C) and so that oneintersection of the lattice aligns with the center of gravity. Thesquares of the lattice are in the shape of squares, those for which thelength of the long axis A of the particles (C) is longer than 20 μm(namely, in the case A≥20 μm) have a length on one side of 2 μm, andthose that are smaller than this (namely, in the case A<20 μm) have alength on one side equal to one-tenth the length of the long axis A(namely, A/10 μm). Among the lattice squares, the number of squares inthe lattice that contain a composite graphite particle (C) is designatedas x. The area of the composite particle (C) relative to the area of thesquare is 50% or less are defined as squares in the lattice that do notcontain the composite graphite particle (C). The number of squares inthe lattice that contain a composite graphite particle (C) and alsocontain a metal particle (B) is designated as y. The values of y/x offive random particles among the 10 particles selected according toCondition 2 for which the lattice is drawn are respectively calculated,and the average value thereof is defined as the degree of dispersion(see FIG. 1 ).

(Condition 4)

Furthermore, particles not composed of graphite (A) and/or metalparticles (B) are excluded from the target particles. Moreover,composite graphite particles (C) that are obviously fragmented or tornare also excluded since they are not suitable as evaluation targets ofthe composite graphite particles (C). In addition, in order to avoidselecting peculiar particles, degree of dispersion is judged to beunmeasurable in the case less than 5 of the 100 composite graphiteparticles (C) contained in the image satisfy the aforementionedextraction conditions.

(Preferable Conditions)

More preferable conditions for selecting the particles extractedaccording to Condition 3 preferably consist of selecting those particlesin which the relationship between the long axis A and short axis Bsatisfies the equation indicated below.|(A/B of particles extracted according to Condition 3)/(average value of(A/B) of all particles extracted according to Condition 1)−1|≤0.5

In addition, although there are no particular limitations on the rangeover which the particles extracted according to Condition 1 areselected, they are preferably extracted from a range of a plurality ofimages measuring 100 μm×150 μm and more preferably from one or moreimages, while preferably extracted from a range of 10 images or less. Inthe case of being unable to extract 100 or more particles within theaforementioned ranges, particles are preferably selected by makingcontrivances to the method used to prepare the polar plate.

Although target composite graphite particles (C) that satisfy theseConditions 1 to 4 and satisfy the degree of dispersion of the presentinvention are only normally required to be present at 5% or more as thenumber of particles in a cross-section over a range of 200 μm×200 μmcross-section of a negative electrode, the number of particles isnormally 30%, more preferably 50%, even more preferably 90% or more andparticularly preferably 99% or more based on the total number of targetcomposite graphite particles (C).

(Physical Properties of Composite Graphite Particles (C))

Although there are no particular limitations on the composite graphiteparticles (C) provided the degree of dispersion of the metal particles(B) in the composite graphite particles (C) is 0.78 or more ascalculated according to the aforementioned measurement method whenobserving cross-sections of the composite graphite particles with ascanning electron microscope (SEM), the composite graphite particles (C)preferably have the properties indicated below.

(1) Tapped Density of Composite Graphite Particles (C)

The tapped density of the composite graphite particles (C) of thepresent invention is normally 0.8 g/cm³ or more, preferably 0.85 g/cm³or more, more preferably 0.88 g/cm³ or more, and even more preferably0.9 g/cm³ or more. On the other hand, the tapped density is normally 1.5g/cm³ or less, preferably 1.4 g/cm³ or less and more preferably 1.35g/cm³ or less.

The demonstrating of a large value for tapped density by the compositegraphite particles (C) of the present invention is an indicator that thecomposite graphite particles (C) exhibit a spherical shape. A smallertapped density is an indicator that the composite graphite particles (C)do not have a sufficiently spherical shape. If tapped density isexcessively low, adequate continuous voids are unable to be secured inthe electrode and mobility of Li ions with electrolytic solutionretained in the voids decreases, thereby resulting in the risk of adecrease in rapid charge-discharge characteristics.

Furthermore, in the case the metal particles (B) are aggregated,compounding with the graphite (A) occurs non-uniformly starting at theaggregated particles, and as a result thereof, there is the possibilityof a decrease in particle fillabiity, or in other words, a decrease intapped density. However, since the metal particles (B) are uniformlydispersed in the composite graphite particles (C) of the presentinvention and maintain favorable binding with graphite, they are thoughtto be particles having high tapped density.

(2) Interplanar Spacing d Value of (002) Plane of Composite GraphiteParticles (C)

The interplanar spacing value d of the (002) plane of the compositegraphite particles (C) of the present invention as determined bywide-angle X-ray diffraction is normally 0.337 nm or less, while on theother hand, since the theoretical value of the interplanar spacing valued of the 002 plane of graphite is 0.335, the interplanar spacing value dof the 002 plane of graphite is normally 0.335 nm or more. In addition,the Lc value of the graphite (A) as determined by wide-angle X-raydiffraction is 90 nm or more and preferably 95 nm or more. Theinterplanar spacing d value of the (002) plane and Lc value of the asdetermined by wide-angle X-ray diffraction being within theaforementioned ranges indicates that the composite graphite particles(C) can serve as a material of a high-capacity electrode.

(3) Volume Mean Particle Diameter (d50) of Composite Graphite Particles(C)

The volume mean particle diameter (d50) of the composite graphiteparticles (C) of the present invention is preferably 40 μm or less andmore preferably 30 μm or less, and normally 1 μm or more, preferably 4μm or more and more preferably 6 μm or more. If the volume mean particlediameter d50 is excessively large, problems such as streaking occurduring coating, while if the volume mean particle diameter d50 isexcessively small, a larger amount of binder is required, resistanceincreases and high current density charge-discharge characteristics tendto decrease.

(4) Raman R Value of Composite Graphite Particles (C)

The ratio of peak intensity in the vicinity of 1360 cm⁻¹ to the peakintensity in the vicinity of 1580 cm⁻¹ in an argon ion laser Ramanspectrum of the composite graphite particles (C) of the presentinvention in the form of the Raman R value is normally 0.05 to 0.4 andpreferably 0.1 to 0.35. If the Raman R value is within these ranges,crystallinity of the surface of the composite graphite particles (C) isorderly and high capacity can be expected.

(5) Specific Surface Area of Composite Graphite Particles (C) DeterminedAccording to BET Method

Specific surface area of the composite graphite particles (C) of thepresent invention as determined according to the BET method is normally20 m²/g or less, preferably 15 m²/g or less, more preferably 10 m²/g orless, even more preferably 8.5 m²/g or less and still more preferably 8m²/g or less, and normally 0.1 m²/g or more, preferably 1 m²/g or more,more preferably 3 m²/g or more, even more preferably 5 m²/g or more andparticularly preferably 6.5 m²/g or more. If the specific surface areais excessively large, since those locations where the composite graphiteparticles (C) contact the nonaqueous electrolytic solution increase whenusing as an active material for a negative electrode, reactivityincreases, the amount of gas generated becomes excessively large, and ittends to be difficult to obtain a preferable battery. If specificsurface area is excessively small, the acceptability of lithium ionsduring charging tends to become poor in the case of using as an activematerial for a negative electrode.

(6) Content of Metal Particles (B) in Composite Graphite Particles (C)

The content of the metal particles (B) in the composite graphiteparticles (C) of the present invention based on the composite graphiteparticles (C) is normally 0.5% by weight or more, preferably 1% byweight or more, more preferably 1.5% by weight or more and even morepreferably 2% by weight or more. In addition, the content of the metalparticles (B) is normally 99% by weight or less, preferably 70% byweight or less, more preferably 50% by weight or less, even morepreferably 30% by weight or less and particularly preferably 25% byweight or less. If the content of the metal particles (B) is withinthese ranges, it becomes possible to obtain adequate capacity, therebymaking this preferable. Furthermore, measurement of the content of themetal particles (B) in the composite graphite particles (C) is carriedout using the method to be subsequently described.

(7) Internal Void Fraction of Composite Graphite Particles (C)

The internal void fraction of the composite graphite particles (C) ofthe present invention based on the graphite (A) of the compositegraphite particles (C) is normally 1% or more, preferably 3% or more,more preferably 5% or more and even more preferably 7% or more. Inaddition, the internal void fraction is normally less than 50%,preferably 40% or less, more preferably 30% or less and even morepreferably 20% or less. If the internal void fraction is excessivelylow, the composite graphite particles (C) tend to be destroyed when themetal particles (B) expand. In addition, these voids may be filled witha substance such as amorphous carbon, graphite material or resin so asto alleviate expansion and contraction of the metal particles (B)capable of alloying with Li. When confirming void fraction by SEM usingcross-sections of the composite graphite particles (C), void fractioncan be calculated using the following equation (2) on 20 randomparticles.

Equation (2)Area of voids within composite graphite particles (C)/area of voids ofgraphite (A)+metal particles (B)+composite graphite particles (C)  (2)

<Production Method of Composite Graphite Particles (C) for NonaqueousSecondary Battery Negative Electrode>

Although there are no particular limitations on the method used toproduce the composite graphite particles (C) for a nonaqueous secondarybattery negative electrode in the present invention provided the methodallows the obtaining of composite graphite particles (C) having theaforementioned properties, an example thereof consists of stirring thegraphite (A), having a comparatively small particle diameter such thatthe value of d50 is 20 μm or less, and the metal particles (B) afteradjusting the concentration such that suitable shearing stress isapplied in the slurry to prepare a slurry containing an aggregate of thegraphite (A) and the metal particles (B), and drying this aggregateslurry so that 5% or more of solvent remains therein, followed bycarrying out spheroidizing treatment with the solvent residue stillcontained in the slurry to efficiently embed the metal particles (B) inthe composite graphite particles (C) in a highly dispersed state. Here,components other than the graphite (A) and the metal particles (B) mayalso be mixed in simultaneously.

More specifically, the production method preferably comprises thefollowing step 1, step 2 and step 3.

Step 1: Step for obtaining a mixture at least containing graphite (A)and metal particles (B).

Step 2: Step for obtaining an aggregate of the graphite (A) and themetal particles (B).

Step 3: Step for applying mechanical energy to the aggregate of step 2and subjecting to spheroidizing treatment.

The following provides a detailed explanation of the production methodof the present invention.

(Step 1: Step for Obtaining Mixture Containing at Least Graphite (A) andMetal Particles (B))

Although examples of the state of the mixture obtained in this stepinclude that in the form of granules, a solid, a block or a slurry, themixture is preferably in the form of a block from the viewpoint of easeof handling.

The mixing ratio of the metal particles (B) to the total amount of thegraphite (A) and the metal particles (B) is normally 1% by weight ormore, preferably 3% by weight or more, more preferably 5% by weight ormore and even more preferably 7% by weight or more. In addition, themixing ratio is normally 95% by weight or less, preferably 90% by weightor less, more preferably 80% by weight or less and even more preferably70% by weight or less. If the mixing ratio is within these ranges,adequate capacity can be obtained, thereby making this preferable. Thegraphite (A) used in this step is normally 1 μm to 50 μm, preferably 2μm to 40 μm and more preferably 5 μm to 30 μm. If within these ranges,composite graphite particles (C) can be produced that are embedded withthe metal particles (B), and composite particles can be obtained inwhich the metal particles (B) are highly dispersed therein.

In addition, in the present step, carbon fine particles may also bemixed in to improve electrical conductivity of the composite graphiteparticles (C), a carbon precursor may be mixed in to inhibit a reactionbetween the metal particles (B) and the nonaqueous electrolyticsolution, or a void forming material in the form of a resin and the likemay be mixed in to alleviate destruction of composite graphite particlescaused by expansion and contraction of the metal particles (B).

In the case of mixing in other materials other than the graphite (A) andmetal particles (B), the mixing ratio of other materials to the totalamount of the graphite (A), the metal particles (B) and the othermaterials is normally 0.1% by weight or more, preferably 0.3% by weightor more, more preferably 0.5% by weight or more and even more preferably0.7% by weight or more. In addition, the mixing ratio is normally 30% byweight or less, preferably 28% by weight or less, more preferably 26% byweight or less and even more preferably 25% by weight or less. If themixing ratio is within these ranges, adequate capacity can be obtained,thereby making this preferable.

In the present step, there are no particular limitations on the methodused to mix the graphite (A), the metal particles (B) and the othermaterials provided a mixture is obtained that at least contains thegraphite (A) and the metal particles (B) capable of alloying with Li.

The mixing method may consist of simultaneously adding the graphite (A),the metal particles (B) capable of alloying with Li and the othermaterials followed by mixing, or sequentially adding each of thesecomponents separately followed by mixing.

An example of a preferable method for obtaining the mixture consists ofusing wet metal particles (B) and mixing with the graphite (A) so as notto allow the metal particles (B) to dry.

Metal particles (B) obtained directly by producing the metal particles(B) using a wet method, metal particles (B) produced using a dry methodmay be dispersed in a dispersion medium prior to mixing with thegraphite (A), or metal particles (B) mixed with other materialsdissolved in a solvent and the like to wet the metal particles (B) maybe used for the wet metal particles (B).

The metal particles (B) that have been wetted in this manner can beuniformly dispersed during mixing since aggregation of the metalparticles (B) is inhibited, and metal particles (B) are easilyimmobilized on the surface of the graphite (A), thereby making thispreferable.

In the present description, in the case of mixing the metal particles(B) in the form of a slurry when mixing the metal particles (B) into thegraphite (A), the solid content of the metal particles (B) in the slurryis normally 10% by weight or more, preferably 15% by weight or more andmore preferably 20% by weight or more, and normally 90% by weight orless, preferably 85% by weight or less and more preferably 80% by weightor less. If the proportion of this solid content is excessively high,the slurry loses fluidity and the metal particles (B) tend to bedifficult to disperse in the graphite (A), while if the proportion ofthe solid content is excessively low, handling tends to become difficultin this step.

In addition, after mixing the graphite (A) and the metal particles (B),the mixture is stirred so as to suitably apply shearing stress to theslurry, and from the viewpoint of facilitating the formation of anaggregate, removing solvent from the mixed slurry or adding thedispersion medium used when wet-crushing the metal particles (B) duringmixing makes it possible to control the solid content.

The non-volatile (NV) ratio at the time of mixing the graphite (A) andthe metal particles (B) when applying shearing stress is preferably 30%or more and more preferably 40% or more. In addition, the non-volatile(NV) ratio is preferably 70% or less and more preferably 60% or less.

In a slurry having an NV ratio within the aforementioned ranges, theslurry is preferably in a state in which it does not flow when leveled,and if the NV ratio is excessively high, excessive shearing stress endsup being applied to the graphite (A), which tends to destroy thegraphite (A) or prevent the formation of aggregates of the graphite (A)and the metal particles (B). In addition, if the NV ratio is excessivelylow, the metal particles (B) undergo migration when removing thesolvent, which tends to lower the degree of dispersion of the compositegraphite particles (C).

(Step 2: Step for Obtaining Aggregates of Graphite (A) and MetalParticles (B))

In the present invention following step 1, aggregates of the graphite(A) and the metal particles (B) are preferably formed by evaporating offand drying the dispersion medium using an evaporater, a dryer and thelike.

Alternatively, aggregates of the graphite (A) and metal particles (B)are preferably formed as a result of suitably applying shearing stressto the slurry by mixing while evaporating the dispersion medium andheating in a high-speed stirrer directly without adding a surplus ofdispersion medium. Furthermore, the aggregates are preferably put into awet state after drying. The content of solvent residue in aggregates ofthe graphite (A) and metal particles (B) based on the total weight ofthe graphite (A) and metal particles (B) is normally 1% by weight ormore, preferably 2% by weight or more and more preferably 3% by weightor more. In addition, the content of solvent residue is normally 40% byweight or less, preferably 30% by weight or less and more preferably 20%by weight or less. If within these ranges, composite particles can beobtained in which the Si compound is highly dispersed. In addition, theamount of solvent residue may be adjusted to a suitable amount bysuitably adding dispersion medium after drying.

Furthermore, there are no particular limitations on the time at whichthe other materials are mixed in order to obtain the mixture andaggregates, and for example, other materials may be added when mixingthe graphite (A) and metal particles (B), the other materials may beadded to wet metal particles (B) or a slurry of metal particles (B), orthe other materials may be added when wet-crushing the metal particles(B). Although the state when mixing the other materials may be a powderor a solution obtained by dissolving in a solvent, a solution ispreferable from the viewpoint of allowing the other materials to beuniformly dispersed.

Among these other materials, the carbon precursor and resin serving as avoid forming material not only fulfill the role of immobilizing themetal particles (B) on the graphite (A), but are also thought to fulfillthe role of preventing dissociation of the metal particles (B) from thegraphite (A) during the spheroidization step. As a result, destructionof the composite graphite particles (C) can be inhibited.

Among the mixtures previously described, a more preferable mixture isobtained by mixing the graphite (A), the metal particles (B) and aresin, and from the viewpoint of being able to uniformly disperse thegraphite (A), the metal particles (B) and the resin for inhibitingdissociation of the metal particles (B) in the mixture, mixing a slurryof the metal particles (B) with a resin for inhibiting dissociation ofthe metal particles (B) dissolved in a solvent followed by mixing thegraphite (A) therein is more preferable for the combination of steps forobtaining this mixture. In addition, at this time, another material inthe form of a carbon precursor may be further mixed in from theviewpoint of being able to inhibit reactivity between the metalparticles (B) and the electrolytic solution, or a void forming materialin the form of a resin may be mixed in to alleviate destruction of thecomposite graphite particles (C) caused by expansion and contraction ofthe metal particles (B).

Although mixing is normally carried out at normal pressure, it can alsobe carried out under reduced pressure or added pressure as desired.Mixing can also be carried out by a batch process or a continuousprocess. In either case, mixing efficiency can be improved by combiningthe use of a device suitable for coarse mixing and a device suitable forfine mixing. In addition, a device may also be used that carries outmixing and immobilization (drying) simultaneously. Although drying cannormally be carried out under reduced pressure or added pressure, dryingis preferably carried out under reduced pressure.

Drying time is normally 5 minutes or more, preferably 10 minutes ormore, more preferably 20 minutes or more and even more preferably 30minutes or more, and normally 2 hours or less, preferably 1.5 hours orless and even more preferably 1 hour or less. If drying time isexcessively long, it leads to increased costs, while if drying time isexcessively short, uniform drying tends to become difficult.

Although varying according to the solvent, drying temperature ispreferably the amount of time that enables drying to be realized in thetimes indicated above.

In addition, the drying temperature is also preferably equal to or lowerthan the temperature at which the resin serving as another material doesnot undergo degeneration.

Examples of devices used for a batch type of mixing device includemixers employing a structure in which two frames rotate while revolving,devices employing a structure in which a single blade carries outstirring and dispersing in a tank in the manner of a high-speed,high-shear mixer in the form of a dissolver or high-viscosity butterflymixer, so-called kneader-type devices that have a structure in which asigma-type or other stirring blade rotates along the lateral surface ofa semi-cylindrical mixing tank, a type of device using biaxial ortriaxial stirring blades, and a so-called bead mill type of devicehaving a rotating disk and a dispersion medium in a container.

In addition, a device employing a structure having a container in whichpaddles rotated by a shaft are housed therein, in which the sides of thecontainer substantially follow the outermost line of the paddlerotation, and are preferably formed into a long catamaran shape, and alarge number of pairs of paddles are arranged in the axial direction ofthe shaft so as to as to bite into mutually opposing sides while able toslide over the sides (such as the KRC Reactor or SC Processormanufactured by Kurimoto, Ltd., the Model TEM manufactured by ToshibaMachine Selmac Co., Ltd., or the Model TEX-K manufactured by Japan SteelWorks Ltd.), or an (exothermic) device employing a structure having asingle internal shaft and a container having a plurality of spade-shapedor serrated paddles attached to the shaft arranged while changing thephase thereof, in which the inside walls thereof substantially followthe outermost line of the paddle rotation, and are preferably formedinto a cylindrical shape (such as the Lodige Mixer manufactured byLodige GmbH, the Flow Share Mixer manufactured by Pacific Machinery &Engineering Co., Ltd., or the DT Dryer manufactured by Tsukishima KikaiCo., Ltd.). A pipeline mixer, continuous bead mill and the like may beused when carrying out mixing with a continuous process. In addition, auniform mixture can also be obtained with a means such as ultrasonicdispersion. Among the aforementioned mixing devices, a device of a typeemploying biaxial or triaxial stirring blades is used preferably fromthe viewpoint of ease of adjusting shearing stress during mixing. Inaddition, a powder step consisting of crushing, shredding orclassification may also be applied to the mixture obtained in this step.

Although there are no particular limitations on the device used forcrushing and shredding, examples of coarse crushers include shearingmills, jaw crushers, impact crushers and cone crushers, examples ofintermediate crushers include roll crushers and hammer mills, andexamples of fine crushers include ball mills, vibration mills, pinmills, mixing mills and jet mills.

Although there are no particular limitations on the device used forclassification treatment, in the case of, for example, dry screening, arotary sieve, shaking sieve, gyrating sieve or vibrating sieve can beused, in the case of dry air classification, a gravity classifier,inertial classifier or centrifugal classifier (such as a classifier orcyclone) can be used, or a wet sieve, mechanical wet classifier,hydraulic classifier, settling classifier or centrifugal wet classifierand the like can be used.

(Step 3: Step for Carrying Out Spheroidizing Treatment by ApplyingMechanical Energy to Aggregate of Step 2)

As a result of going through step 3, aggregates containing the graphite(A) and the metal particles (B) can be further compounded, resulting inthe attainment of high tapped density and a high degree of dispersion,thereby making this preferable.

In other words, although an example of a production method for obtainingthe composite graphite particles (C) of the present invention consistsof carrying out spheroidizing treatment on aggregates containing themetal particles (B) on the surface of the graphite (A) obtained in theaforementioned step 2 (to also be simply referred to as aggregates inthe present description), in the present invention in particular,production conditions as subsequently described are preferably suitablyset so that the metal particles (B) are made to be present in voids ofthe graphite (A) within a prescribed range. As a result of carrying outspheroidizing treatment in a state of containing solvent residue,adhesion between the graphite (A) and metal particles (B) in theaggregates increases, and since the metal particles (B) can be inhibitedfrom separating from the aggregates and aggregating, composite graphiteparticles (C) tend to be obtained that have a high degree of dispersion.

Furthermore, spheroidizing treatment specifically refers to treatmentthat utilizes mechanical energy (mechanical action such as impactcompression, friction or shearing force). More specifically, treatmentusing a hybridization system is preferable. This system has a rotorhaving a large number of blades that apply mechanical action such asimpact compression, friction or shearing force, a large air flow isgenerated by rotation of the rotor, considerable centrifugal force isapplied to the graphite (A) in the mixture obtained in theaforementioned step 1 as a result thereof, and the aggregates obtainedin step 2 can be compounded as a result of collisions between graphite(A) in the aggregates obtained in the aforementioned step 2 and betweenthe graphite (A) present in the aggregates obtained the aforementionedstep 2, the walls and the blades.

An example of a device used for spheroidizing treatment consists of adevice that performs surface treatment by having a rotor installed witha large number of blades housed in a casing, and applying mechanicalaction such as impact compression, friction or shearing force tographite present in the mixture obtained in step 1 that has beenintroduced therein by rotating the rotor at high speeds. Althoughexamples thereof include a dry ball mill, wet bead mill, planetary ballmill, vibration ball mill, mechanofusion system, Agromaster (HosokawaMicron Corp.), Hybridization System, Micros, Miralo (Nara Machinery Co.,Ltd.), CF Mill (Ube Industries, Ltd.) and Theta Composer (Tokuju Corp.),preferable examples of devices include a dry ball mill, wet bead mill,planetary ball mill, vibration ball mill, mechanofusion system,Agromaster (Hosokawa Micron Corp.), Hybridization System, Micros, Miralo(Nara Machinery Co., Ltd.), CF Mill (Ube Industries, Ltd.), ThetaComposer (Tokuju Corp.) and pulverizer. Among these, the HybridizationSystem manufactured by Nara Machinery Co., Ltd. is particularlypreferable.

Furthermore, although the graphite (A) in the aggregates obtained in theaforementioned step 2 subjected to spheroidizing treatment may also bethat which has already been subjected to a certain spheroidizingtreatment under the conditions of a conventional method, flake graphite(A) is preferable from the viewpoint of dispersibility. In addition, theaggregates obtained in step 2 may also be repeatedly subjected tomechanical action by circulating or by going through this step aplurality of times.

Although spheroidizing treatment is carried out using this type ofdevice, when carrying out this treatment, spheroidizing treatment iscarried out at a rotor rotating speed of normally 2000 rpm or more,preferably 4000 rpm or more, more preferably 5000 rpm or more, even morepreferably 6000 rpm or more and particularly preferably 6500 rpm ormore, and normally at 9000 rpm or less, preferably 8000 rpm or less,more preferably 7500 rpm or less and even more preferably 7200 rpm orless, for a duration of normally 30 seconds or more, preferably 1 minuteor more, more preferably 1 minute 30 seconds or more, even morepreferably 2 minutes or more and particularly preferably 2 minutes 30seconds or more, and normally for 60 minutes or less, preferably 30minutes or less, more preferably 10 minutes or less and even morepreferably 5 minutes or less.

Furthermore, if the rotor rotating speed is excessively slow, treatmentfor obtaining composite particles becomes ineffective and there is thepossibility of an insufficient increase in degree of dispersion, whileif the rotating speed is excessively fast, the effect of crushing theparticles becomes more effective than the treatment for obtainingcomposite particles, resulting in the possibility of the particlescollapsing and the degree of dispersion decreasing. Moreover, if theduration of spheroidizing treatment is excessively short, particlediameter is unable to be made sufficiently small while achieving a highdegree of dispersion, while if the duration is excessively long, thegraphite (A) in the aggregates obtained in step 2 is crushed, therebyresulting in the possibility of being unable to achieve the object ofthe present invention.

Furthermore, classification treatment may also be carried out on theresulting composite graphite particles (C). In the case the resultingcomposite graphite particles (C) are not within the range of thephysical properties defined in the present invention, they can be madeto be within a desired range of physical properties by repeatedlysubjecting to classification treatment (normally for 2 to 10 times andpreferably for 2 to 5 times). Although examples of classificationinclude dry classification (air classification, screening) and wetclassification, dry classification, and particularly air classification,is preferable in terms of cost and productivity.

The composite graphite particles (C) can be produced according to theproduction method described above.

<Carbonaceous Material-Coated Composite Graphite Particles>

Although the composite graphite particles (C) used in the presentinvention are obtained in the manner described above, the compositegraphite particles (C) preferably contain a carbonaceous material, and apreferable specific aspect thereof consists of composite graphiteparticles having a carbonaceous material at least partially coated onthe surface thereof (to be referred to as carbonaceous material-coatedcomposite graphite particles).

Furthermore, in the present description, although the carbonaceousmaterial-coated composite graphite particles are distinguished from thecomposite graphite particles (C) for the sake of convenience, thecarbonaceous material-coated composite graphite particles areinterpreted as being included in the composite graphite particles (C).

(Production Method of Carbonaceous Material-Coated Composite GraphiteParticles)

The carbonaceous material-coated composite graphite particles can beproduced by going through a step 4 following the previously describedstep 3.

Step 4: Step for coating the composite graphite particles subjected tospheroidizing treatment in step 3 with a carbonaceous material.

The following provides a detailed explanation of step 4.

(Step 4: Step for Coating Composite Graphite Particles Subjected toSpheroidizing Treatment in Step 3 With Carbonaceous Material)

Carbonaceous Material

Examples of the carbonaceous material include amorphous carbon andgraphitizable carbon depending on differences in the heating temperaturein the production method thereof to be subsequently described. Amongthese, amorphous carbon is preferable from the viewpoint of acceptableof lithium ions.

More specifically, the aforementioned carbonaceous material can beobtained by heat treating a carbon precursor thereof in the mannerdescribed below. Carbon precursors explained in the previously describedsection on other materials are preferably used for the aforementionedcarbon precursor.

Coating Treatment

In the coating treatment, a carbon precursor for obtaining acarbonaceous material is used as a coating material for coating thecomposite graphite particles obtained in the aforementioned step 2, andcoated graphite is obtained by their mixing and firing.

If the firing temperature is normally 600° C. or higher, preferably 700°C. or higher and more preferably 900° C. or higher, and normally 2000°C. or lower, preferably 1500° C. or lower and more preferably 1200° C.or lower, amorphous carbon is obtained as carbonaceous material. On theother hand, if heat treatment is carried out at a firing temperature ofnormally 2000° C. or higher, preferably 2500° C. or higher and normally3200° C. or lower, graphitizable carbon is obtained as carbonaceousmaterial. The amorphous carbon refers to carbon having a low degree ofcrystallinity, while the graphitizable carbon refers to carbon having ahigh degree of crystallinity.

During coating treatment, the aforementioned composite graphiteparticles (C) are used as a core material, the precursor carbon forobtaining a carbonaceous material is used as a coating raw material, andthese are then mixed and fired to obtain carbonaceous material-coatedcomposite graphite particles.

Furthermore, in the case of coating other resins and the like, the resinis preferably dissolved in a solvent and mixed with the composite firedin step 4 followed by blending well, washing and drying.

Mixing of Metal Particles (B) and Carbon Fine Particles

The coating layer may contain the metal particles (B) and carbon fineparticles previously explained in the section on other materials.

Other Steps

In addition, the carbonaceous material-coated composite graphiteparticles that have gone through the aforementioned steps may undergopowder processing such as the crushing, shredding and classificationtreatment described in step 1.

The carbonaceous material-coated composite graphite particles of thepresent invention can be produced according to the production methoddescribed above.

(Physical Properties of Carbonaceous Material-Coated Composite GraphiteParticles)

Although the carbonaceous material-coated composite graphite particlesdemonstrate the same physical properties as those of the previouslydescribed composite graphite particles, those preferable physicalproperties of the carbonaceous material-coated composite graphiteparticles that are particularly altered by coating treatment aredescribed below.

d Value of Interplanar Spacing of (002) Plane

The d value of interplanar spacing of the (002) plane of thecarbonaceous material-coated composite graphite particles as determinedby wide-angle X-ray diffraction is normally 0.336 nm or more, preferably0.337 nm or more, more preferably 0.340 nm or more and even morepreferably 0.342 nm or more. In addition, the d value is normally lessthan 0.380 nm, preferably 0.370 nm or less and more preferably 0.360 nmor less. An excessively large d002 value results in the demonstration oflow crystallinity and cycling characteristics tend to decrease, while ifthe d002 value is excessively small, it becomes difficult to obtain theeffect of compounding the carbonaceous material.

Coverage Factor

Although the carbonaceous material-coated composite graphite particlesare coated with amorphous carbon or graphitizable carbon, coating withamorphous carbon is preferable from the viewpoint of acceptability oflithium ions, and the coverage factor thereof is normally 0.5% or more,preferably 1% or more, more preferably 3% or more, even more preferably4% or more, particularly preferably 5% or more and most preferably 6% ormore, and normally 30% or less, preferably 25% or less, more preferably20% or less, even more preferably 15% or less, particularly preferably10% or less and most preferably 8% or less. If this content isexcessively high, the amorphous carbon portion of the negative electrodematerial becomes excessively large and reversible capacity whenincorporating in a battery tends to decrease. If the content isexcessively low, in addition to amorphous carbon sites not beinguniformly coated on the graphite particles serving as cores, solidgranulation does not occur and particle diameter tends to be excessivelysmall during crushing after firing.

Furthermore, the content of carbide derived from organic compounds inthe ultimately obtained carbon material for an electrode (coveragefactor) can be calculated using the following equation (3) based on theamount of raw material carbon used, the amount of organic compound, andthe residual carbon ratio measured according to the micro method incompliance with JIS K2270-02 (2009).Coverage factor of carbide derived from organic compound (%)=(weight oforganic compound×residual carbon ratio×100)/{weight of raw materialcarbon+(weight of organic compound×residual carbon ratio)}  Equation (3)

<Other Mixtures>

Although the composite graphite particles (C) of the present inventioncan be used alone as an active material for a nonaqueous secondarybattery negative electrode, it is preferably used for the activematerial of a nonaqueous secondary battery negative electrode by furthercontaining one or more types of materials selected from the groupconsisting of natural graphite, artificial graphite, vapor-grown carbonfibers, electrically conductive carbon black, carbonaceousmaterial-coated graphite, resin-coated graphite, amorphous carbon andmaterials obtained by subjecting these materials to suitable treatmentthat have different forms and physical properties from theaforementioned composite graphite particles (C). Among these, an activematerial for a nonaqueous secondary battery negative electrodecontaining the composite graphite particles (C) of the present inventionand one or more types of materials selected from the group consisting ofnatural graphite, artificial graphite, carbonaceous material-coatedgraphite, resin-coated graphite and amorphous carbon is more preferable.

As a result of suitably selecting and mixing carbonaceous particleshaving different forms and physical properties, cycling characteristicscan be improved as a result of improving electrical conductivity, chargeacceptance can be improved, irreversible capacity can be decreased androllability can be improved.

The mixing ratio of the composite graphite particles (C) to the totalamount of composite graphite particles (C) and carbonaceous particleshaving a form and properties different therefrom is normally 1% byweight or more, preferably 1.5% by weight or more, more preferably 2% byweight or more, and even more preferably 2.5% by weight or more. Inaddition, the mixing ratio is normally 99% by weight or less, preferably95% by weight or less, more preferably 90% by weight or less and evenmore preferably 85% by weight or less.

If the amount of composite graphite particles (C) is excessively high,volume expansion accompanying charging and discharging increases,deterioration of capacity becomes prominent and press load may decrease.In addition, if the amount of composite graphite particles (C) isexcessively low, adequate capacity tends to not be obtained.

Among those carbonaceous particles having different form and properties,highly purified flake graphite or spheroidized graphite, for example,can be used as natural graphite.

Particles obtained by compounding coke powder or natural graphite with abinder or particles obtained by firing and graphitizing single graphiteprecursor particles while still in the form of a powder, for example,can be used as artificial graphite.

Particles obtained by coating a carbonaceous material precursor ontonatural graphite or artificial graphite followed by firing, or particlesobtained by coating a carbonaceous material onto the surface of naturalgraphite or artificial graphite, for example, can be used ascarbonaceous material-coated graphite.

Particles obtained by coating a polymeric material onto natural graphiteor artificial graphite followed by drying, for example, can be used asresin-coated graphite, while particles obtained by firing bulk mesophaseor particles obtained by subjecting a carbonaceous material precursor toinfusibilization treatment followed by firing, for example, can be usedas amorphous carbon.

<Negative Electrode for Nonaqueous Secondary Battery>

In order to fabricate a negative electrode using an active material fora nonaqueous secondary battery negative electrode containing thecomposite graphite particles (C) according to the present invention, amixture obtained by incorporating a binder resin in an active materialfor a nonaqueous secondary battery negative electrode is formed into aslurry with water or organic solvent, followed by coating onto a currentcollector after adding a thickener as necessary and drying.

A binder resin that is stable in nonaqueous electrolytic solution and isnon-water-soluble is preferably used for the binder resin. Examplesthereof include rubber-like polymers such as styrene, butadiene rubber,isoprene rubber or ethylene-propylene rubber, synthetic resins such aspolyethylene, polypropylene, polyethylene terephthalate or aromaticpolyamides, thermoplastic elastomers such as styrene-butadiene-styreneblock copolymers and hydrogenated products thereof,styrene-ethylene-butadiene-styrene copolymers orstyrene-isoprene-styrene block copolymers and hydrogenated productsthereof, soft resin-like polymers such as syndiotactic1,2-polybutadiene, ethylene-vinyl acetate copolymer or copolymers ofethylene and α-olefins having 3 to 12 carbon atoms, and fluorinatedpolymers such as polyvinylidene fluoride, polypentafluorpropylene orpolyhexafluoropropylene. Examples of organic solvents include NMP andDMF.

The binder resin is used at preferably 0.1% by weight and morepreferably 0.2% by weight based on the active material for a nonaqueoussecondary battery negative electrode. By making the ratio of binderresin to active material for a nonaqueous secondary battery negativeelectrode to be 0.1% by weight or more, mutual adhesion between activematerials for a nonaqueous secondary battery negative electrode andadhesion between the composite graphite particles and current collectorare adequate, thereby making it possible to prevent reductions inbattery capacity and exacerbation of cycling characteristics caused bythe active material for a nonaqueous secondary battery negativeelectrode dissociating from the negative electrode. The binder resin ispreferably used at 10% by weight at most and preferably at 7% by weightor less.

Examples of thickeners added to the slurry include water-solublecelluloses such as carboxymethyl cellulose, methyl cellulose,hydroxyethyl cellulose or hydroxypropyl cellulose, as well as polyvinylalcohol and polyethylene glycol. Among these, carboxymethyl cellulose ispreferable. The thickener is used at preferably 0.2% by weight to 10% byweight and more preferably at 0.5% by weight to 7% by weight based onthe composite graphite particles.

A material conventionally known to be able to be used in thisapplication is used for the negative electrode current collector, andexamples thereof include copper, copper alloys, stainless steel, nickel,titanium and carbon. The shape of the current collector is normally thatof a sheet, and preferably has surface irregularities on the surfacethereof and preferably uses a metal mesh or perforated metal.

After having coated a slurry of the active material for a nonaqueoussecondary battery negative electrode and the binder resin onto thecurrent collector followed by drying, the coated slurry is preferablypressured to increase the density of the negative electrode activematerial layer formed on the current collector, resulting in an increasein the battery capacity per unit volume of the negative electrode activematerial layer. The density of the negative electrode active materiallayer is preferably 1.2 g/cm³ or more and more preferably 1.3 g/cm³ ormore, and preferably 1.9 g/cm³ or less and more preferably 1.8 g/cm³ orless. As a result of making the density of the negative electrode activematerial layer to be 1.2 g/cm³ or more, decreases in battery capacityaccompanying increases in electrode thickness can be prevented. As aresult of making the density of the negative electrode active materiallayer to be 1.8 g/cm³ or less, reductions in the amount of electrolyticsolution retained in voids accompanying reductions in inter-particlevoids in the electrode, and decreases in rapid charge-dischargecharacteristics attributable to decreased mobility of alkaline ions suchas lithium (Li) ions, can be prevented.

<Nonaqueous Secondary Battery>

The nonaqueous secondary battery according to the present invention canbe fabricated in accordance with ordinary methods with the exception ofusing the aforementioned negative electrode. Lithium transition metalcomplex oxides such as lithium cobalt complex oxide having a basiccomposition represented by LiCoO₂, a lithium nickel complex oxiderepresented by LiNiO₂ or a lithium manganese complex oxide representedby LiMnO₂ or LiMn₂O₄, transition metal oxides such as manganese dioxide,mixed complex oxides thereof as well as TiS₂, FeS₂, Nb₃S₄, Mo₃S₄, CoS₂,V₂O₅, CrO₃, V₃O₃, FeO₂, GeO₂ or LiNi_(0.33)Mn_(0.33)O₂ may be used forthe positive electrode material.

The positive electrode can be fabricated by forming a mixtureincorporating a binder resin in these positive electrode materials intoa slurry with a suitable solvent, and coating the slurry onto a currentcollector followed by drying. Furthermore, the slurry preferablycontains an electrically conductive material such as acetylene black orKetjen black.

In addition, a thickener may also be contained as desired. Examples ofthickeners and binder resins that may be used include commonly knownthickeners and binders used in this application, such as those used tofabricate a negative electrode.

The mixing ratio of the electrically conductive material based on thepositive electrode material is normally 0.5% by weight to 20% by weightand particularly preferably 1% by weight to 15% by weight. The mixingratio of the thickener is preferably 0.2% by weight to 10% by weight andmore preferably 0.5% by weight to 7% by weight. The mixing ratio of thebinder resin when forming a slurry with water is preferably 0.2% byweight to 10% by weight and more preferably 0.5% by weight to 7% byweight. In addition, the mixing ratio of a binder resin such as NMP whenforming a slurry with an organic solvent is preferably 0.5% by weight to20% by weight and more preferably 1% by weight to 15% by weight.

Aluminum, titanium, zirconium, hafnium, niobium, tantalum or an alloythereof may be used for the positive electrode current collector.

Among these, aluminum, titanium, tantalum or an alloy thereof is usedpreferably, and aluminum and/or an alloy thereof are used mostpreferably.

An electrolytic solution obtained by dissolving various lithium salts ina conventionally known nonaqueous solvent can be used for theelectrolytic solution. Examples of nonaqueous solvents include cycliccarbonates such as ethylene carbonate, fluoroethylene carbonate,propylene carbonate, butylene carbonate or vinylene carbonate, linearcarbonates such as dimethyl carbonate, ethyl methyl carbonate or diethylcarbonate, cyclic esters such as γ-butyrolactone, cyclic ethers such ascrown ether, 2-methyltetrahydrofuran, tetrahydrofuran,1,2-dimethyltetrahydrofuran or 1,3-dioxolane, and linear ethers such as1,2-dimethoxyethane. Normally several of these solvents are used incombination. Among these, cyclic carbonates, linear carbonates and thoseused in combination with other solvents are preferable.

In addition, compounds such as vinylene carbonate, vinyl ethylenecarbonate, succinic anhydride, maleic anhydride, propane sultone ordiethylsulfone, or difluorophosphates in the manner of lithiumdifluorophosphate, may also be added. Moreover, overcharge preventiveagents such as diphenyl ether or cyclohexylbenzene may also be added.

Examples of electrolytes dissolved in nonaqueous solvent that may beused include LiClO₄, LiPF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂,LiN(CF₃CF₂SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂) and LiC(CF₃SO₂)₃. Theconcentration of electrolyte in the electrolytic solution is normally0.5 mol/l to 2 mol/l and preferably 0.6 mol/l to 1.5 mol/l.

A porous sheet composed of a polyolefin such as polyethylene orpolypropylene, or a non-woven fabric, is preferably used for theseparator interposed between the positive electrode and negativeelectrode.

The capacity ratio of negative electrode/positive electrode in thenonaqueous secondary battery according to the present invention ispreferably designed to be 1.01 to 1.5 and more preferably designed to be1.2 to 1.4.

The nonaqueous secondary battery is preferably a lithium ion secondarybattery provided with a positive electrode and negative electrode,capable of occluding and releasing lithium ions, and an electrolyte.

EXAMPLES

Although the following provides a more detailed explanation of specificaspects of the present invention through examples thereof, the presentinvention is not limited by these examples.

Furthermore, measurement of the volume mean particle diameter (d50), BETspecific surface area, Si content of the composite graphite particlesand Si degree of dispersion of the composite graphite particles in thepresent description was carried out according to the methods indicatedbelow.

Volume Mean Particle Diameter (d50)

Volume mean particle diameter (d50) was determined by adding about 20 mgof carbon powder to 1 ml of a 2% aqueous solution of polyoxyethylene(20) sorbitan monolaurate followed by dispersing this in about 200 ml ofion exchange water, and measuring volume particle size distribution ofthe resulting dispersion using a laser diffraction particle size:distribution analyzer (LA-920, Horiba, Ltd.) to determine mediandiameter (d50). Measurement conditions consisted of ultrasonicdispersion for 1 minute, ultrasonic intensity of 2, circulating speed of2 and relative refractive index of 1.50.

BET Specific Surface Area (SA)

BET specific surface area (SA) was measured using the Tristar II300manufactured by Micromeritics Japan G.K. After vacuum drying for 1 hourat 150° C., BET specific surface area was measured according to the BETmultipoint method using nitrogen gas adsorption (by measuring 5 pointswithin a relative pressure range of 0.05 to 0.30).

Tapped Density of Composite Graphite Particles

Tapped density of the composite graphite particles was determined bydropping a sample through a sieve having an opening size of 300 μm intoa cylindrical tapping cell having a diameter of 1.6 cm and volumecapacity of 20 cm³ using a powder density tester and completely fillingthe cell, followed by tapping 1000 times at a stroke length of 10 mm anddetermining density from the volume and sample weight at that time.

Si Content of Composite Graphite Particles

Si content of the composite graphite particles was determined bycompletely melting a sample (composite graphite particles) with basefollowed by dissolving with water and bringing to a constant volume,measuring with an inductively-coupled plasma emission spectrometer(Ultima 2C, Horiba, Ltd.) and calculating the amount of Si from acalibration curve. Subsequently, the Si content of the compositegraphite particles was calculated by dividing the amount of Si by theweight of the composite graphite particles.

Degree of Dispersion of Si in Composite Graphite Particles

The degree of dispersion of Si in the composite graphite particles wasmeasured in the manner indicated below. A polar plate similar to thepolar plate used to fabricate a battery for evaluating performance asdescribed below was used for the polar plate containing compositegraphite particles. First, a cross-section of the electrode wasprocessed using a cross-section polisher (IB-09020CP, JEOL Ltd.). Thegraphite (A) and Si were mapped using reflected electron images whileobserving the processed electrode cross-section with an SEM (SU-70,Hitachi High-Technologies Corp.). Furthermore, conditions for acquiringthe SEM cross-section consisted of an accelerating voltage of 3 kV andmagnification factor of 1000×, and images were obtained over a rangecapable of acquiring a single particle at a resolution of 256 dpi.Subsequently, 100 particles were extracted using two SEM imagesmeasuring 150 μm×100 μm in accordance with the measurement method andconditions for degree of dispersion described above, and 10 particleswere selected therefrom that satisfy the aforementioned degree ofdispersion measurement conditions to calculate degree of dispersionusing the y/x values of 5 random particles.

Example 1

(Preparation of Composite Graphite Particles (C))

(Step 1)

First, metal particles (B) in the form of polycrystalline Si having amean particle diameter d50 of 1 μm were crushed to a mean particlediameter d50 of 0.2 μm with an LMZ10 (Ashizawa Finetech Ltd.) to preparean Si slurry (I). 300 g of this Si slurry (I) (solid content: 40% byweight) were added to 1500 g of NMP without allowing to dry followed bystirring using a mixing stirrer (Dalton Co., Ltd.). Next, 1000 g ofgraphite (A) in the form of natural flake graphite (mean particlediameter d50: 9 μm) were added followed by mixing using a mixing stirrerat an NV ratio of 40% by weight to obtain a slurry (II) in which Sicompound particles and graphite were uniformly dispersed therein.

(Step 2)

This slurry (II) was dried under reduced pressure at 150° C. until thesolvent residue became 10% by weight based on the charged amounts of thegraphite (A) and metal particles (B). The resulting aggregates werecrushed with a mill having a hammer-shaped head (IKA Corp.).

(Step 3)

The crushed aggregate was charged into a hybridization system (NaraMachinery Co., Ltd.) followed by subjecting to spheroidizing treatmentby circulating or retaining in a device at a rotor rotating speed of7000 rpm for 180 seconds to obtain composite graphite particles (C)having Si compound particles enclosed therein.

(Step 4)

The resulting composite graphite particles (C) containing Si compoundparticles were mixed with coal-based heavy oil so as to have a coveragefactor after firing of 7.5% followed by kneading and dispersing with abiaxial kneader. The resulting dispersion was introduced into a firingfurnace and fired for 1 hour at 1000° C. in a nitrogen atmosphere. Thefired block was crushed under conditions of a rotating speed of 3000 rpmusing the aforementioned mill followed by classifying with a vibrationsieve having an opening size of 45 μm to obtain composite graphiteparticles (C) coated with amorphous carbon.

The mean particle diameter (d50), BET specific surface area, Si content,Si degree of dispersion and tapped density of the resulting compositegraphite particles are described in Table 1. Furthermore, across-sectional SEM image of the composite graphite particles (C) isshown in FIG. 2 .

When the cross-sectional structure was observed in the cross-sectionalSEM image, the degree of dispersion of Si in the composite graphiteparticles (C) was 0.96 and Si dispersibility was confirmed to be high.

Evaluation of Battery Characteristics

A battery evaluation (consisting of evaluations of discharge capacity,initial charge-discharge efficiency and discharge C ratecharacteristics) was carried out on the resulting composite graphiteparticles (C). The results of the battery evaluation are shown in Table1.

(Fabrication of Battery for Performance Evaluation)

97.5% by weight of the aforementioned composite graphite particles (C),1% by weight of a binder in the form of carboxymethyl cellulose (CMC),and 1.5% by weight of a 48% by weight aqueous dispersion ofstyrene-butadiene rubber (SBR) were kneaded with a hybridizing mixer toobtain a slurry. This slurry was coated onto rolled copper foil having athickness of 18 μm to a basis weight of 4 mg/cm² to 5 mg/cm² by bladecoating followed by drying.

Subsequently, the coated copper foil was roll-pressed to a density ofthe negative electrode active material layer of 1.3 g/cm³ to 1.5 g/cm³with a 250 mm diameter roll press equipped with a load cell followed bypunching into a circular shape having a diameter of 12.5 mm and vacuumdrying for 2 hours at 110° C. to obtain a negative electrode forevaluation. The aforementioned negative electrode and Li foil serving asthe counter electrode were superimposed with a separator impregnatedwith an electrolytic solution interposed there between to fabricate abattery for charge-discharge testing. An electrolytic solution obtainedby dissolving fluoroethylene carbonate to 10% by weight and LiPF₆ to 1.2mol/l in a mixed solvent of ethylene carbonate and ethyl methylcarbonate (weight ratio: 3/7) was used for the electrolytic solution.

(Discharge Capacity/Initial Charge-Discharge Efficiency)

First, the aforementioned battery was charged to 5 mV relative to thepositive electrode and negative electrode at a current density of 0.8mA/cm², further charged at a constant voltage of 5 mV until the currentvalue reached 0.08 mA, and after doping the negative electrode withlithium, the battery was discharged to 1.5 V relative to the positiveelectrode and negative electrode at a current density of 0.8 mA/cm². Theweight of the negative active material was determined by subtracting theweight of copper foil punched out to the same area as the negativeelectrode from the weight of the negative electrode, and the dischargecapacity of the first cycle was divided by this weight of the negativeelectrode active material to determine the initial charge-dischargecapacity per unit weight which was then used as discharge capacity.

Next, the battery was charged and discharged under the same conditions,and after determining the discharge capacity per unit weight of thesecond cycle, initial charge-discharge efficiency was determined usingequation (4) indicated below.Initial charge-discharge efficiency (%)={discharge capacity of secondcycle (mAh/g)/(discharge capacity of first cycle (mAh/g)}×100  Equation(4)

(Discharge C Rate Characteristics)

The battery for which initial charge-discharge efficiency was determinedabove was charged to 5 mV relative to the positive electrode andnegative electrode at a current density of 0.8 mA/cm², and furthercharged at a constant voltage of 5 mV until the current value reached0.08 mA. This battery was then discharged to 1.5 V relative to thepositive electrode and negative electrode at a current density of 0.8mA/cm². Next, after charging this battery in the same manner, thebattery was discharged to a current density of 4.0 mA/cm². Discharge Crate was determined according to equation (5) below using each dischargecapacity.Discharge C rate (%)=capacity during discharge at 4.0 mA/cm²/capacityduring discharge at 0.8 mA/cm²×100  Equation (5)

Example 2

Composite particles were obtained using the same method as Example 1with the exception of adding 30 g of polyacrylonitrile to inhibitdissociation of metal particles when mixing the slurry (II), and usingnatural flake graphite (mean particle diameter d50: 13 μm) for thegraphite (A). The properties of the resulting composite graphiteparticles and the results of battery evaluation are described in Table1.

Example 3

Composite particles were obtained using the same method as Example 1with the exception of using natural flake graphite (mean particlediameter d50: 15 μm) for the graphite (A). The properties of theresulting composite graphite particles and the results of batteryevaluation are described in Table 1.

Comparative Example 1

Composite particles were obtained using the same method as Example 1with the exception of using a slurry obtained by additionally drying theslurry (II) used in step 1 having graphite uniformly dispersed thereinso that solvent residue became 0.5% by weight or less based on thecharged amounts of the graphite (A) and metal particles (B). Theproperties of the resulting composite graphite particles and the resultsof battery evaluation are described in Table 1.

Comparative Example 2

Composite particles were obtained using the same method as Example 1with the exception of changing the size of the flake graphite used instep 1 to 45 μm. The properties of the resulting composite graphiteparticles and the results of battery evaluation are described inTable 1. In addition, an SEM image of the cross-section of the compositegraphite particles is shown in FIG. 3 .

Comparative Example 3

The Si slurry (I) (solid content: 40% by weight) was dried to obtain Sicompound particles. After mixing 190 g of the Si compound particles and1000 g of spherical graphite particles (mean particle diameter d50: 16μm), mixing with coal tar pitch and firing were carried out using thesame procedure as that of Example 1. The properties of the resultingcomposite graphite particles and the results of battery evaluation aredescribed in Table 1.

Comparative Example 4

Coal tar pitch was added in step 1 in an amount such that the coveragefactor after firing was 7.5 parts by weight. Subsequently, after firingfor 2 hours at 400° C. in a nitrogen atmosphere, the particles werecrushed to a particle size of 10 μm. Following crushing, 200 g of thefired powder were treated with a mechanofusion system (Hosokawa MicronCorp.) for 20 minutes at 2000 rpm. After firing at 1000° C., the firedblock was crushed using the previously described mill under conditionsof a rotating speed of 3000 rpm followed by classifying with a vibrationsieve having an opening size of 45 μm to obtain composite graphiteparticles (C) coated with amorphous carbon. The properties of theresulting composite graphite particles and the results of batteryevaluation are described in Table 1.

TABLE 1 Initial Degree charge- of discharge d50 SA TAP disper- Dischargeefficiency (μm) (m²/g) (g/cm³) sion C rate (%) Example 1 11 7.6 0.9230.96 0.993 87.2 Example 2 11 9.2 0.984 0.90 0.993 85.9 Example 3 15 6.80.98 0.80 0.988 89.4 Comparative 10 6.2 0.875 0.75 0.982 86.8 Example 1Comparative 20 13 1.05 0.66 0.990 86.4 Example 2 Comparative 19 8.3 1.030.4 0.855 85.6 Example 3 Comparative 10 6.8 0.695 0.56 0.988 86.6Example 4

As shown in Table 1, the composite graphite particles (C) of the presentinvention were confirmed to have high discharge C rate characteristics.In addition, they were also confirmed to have high initialcharge-discharge efficiency. In particular, Examples 1 and 2 havingfavorable degrees of dispersion were confirmed to demonstrate extremelyhigh discharge C rate characteristics. In addition, Examples 1 and 3, inwhich degree of dispersion and specific surface area were withinpreferable ranges, were determined to demonstrate superior initialcharge-discharge efficiency while maintaining a high C rate. This isthought to be because, since the composite graphite particles (C) of thepresent invention have Si present within the particles in a highlydispersed state, there is little interruption of electron conductivepaths caused by local expansion, thereby resulting in balance betweendischarge C rate characteristics and initial charge-discharge efficiencyat a high level.

Although a detailed explanation of the present invention has beenprovided with reference to specific embodiments, it should be clear to aperson with ordinary skill in the art that various alterations andmodifications can be added without deviating from the spirit and scopeof the present invention.

INDUSTRIAL APPLICABILITY

A nonaqueous secondary battery provided with an electrode using thecomposite graphite particles of the present invention has high capacityand demonstrates improved initial charge-discharge characteristics andcharge-discharge efficiency, thereby making it industrially useful as aresulting of being able to satisfy the properties required ofapplications in cell phones, power tools and electric automobiles inrecent years.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   (1) Solid line: Extracted composite graphite particle (C)    -   (2) Metal particle (B)    -   (3) Center of gravity    -   (4) Dotted line: Long axis of extracted particle    -   (5) Dotted line: Short axis of extracted particle    -   (6) Lattice having square-shaped squares

The invention claimed is:
 1. Composite graphite particles comprisinggraphite and metal particles, wherein the degree of dispersion of themetal particles in the composite graphite particles is 0.78 or more;wherein the internal void fraction of the composite graphite particlesis 3% or more and 40% or less; wherein the degree of dispersion isdefined by the following measurement method: when a lattice is drawn inthe form of a grid having a length of 2 μm per side (however, length perside A/10 μm in the case the length of the long axis <20 μm) for each ofthe images of the scanning electron microscope (SEM) of thecross-sections of 10 composite graphite particles satisfying thefollowing condition:|0.5×(A+B)−R|≤3, wherein, A represents the length of the long axis (μm),B represents the length of the short axis (μm), and R represents themean particle diameter d50 (μm), the number of squares in the latticethat contain composite graphite particles are defined as x, and thenumber of squares in the lattice containing composite graphite particlesthat also contain metal particles are defined as y, then the values ofy/x for any 5 particles are calculated, and the average value thereof isdefined as the degree of dispersion; wherein the internal void fractionis defined by the following measurement method: when confirming voidfraction by SEM using cross-sections of the composite graphiteparticles, void fraction is calculated using the following equation (2)on 20 random particles:Area of voids within composite graphite particles/(area of voids ofgraphite+metal particles+composite graphite particles)  (2).
 2. Thecomposite graphite particles according to claim 1, wherein the tappeddensity of the composite graphite particles for a negative electrode is0.8 g/cm³ or more.
 3. The composite graphite particles according toclaim 1, wherein the metal particles are contained at 1% by weight to30% by weight of the composite graphite particles.
 4. The compositegraphite particles according to claim 1, wherein the specific surfacearea as determined by the BET method is 0.1 m²/g to 20 m²/g.
 5. Anactive material for a nonaqueous secondary battery negative electrodecomprising the composite graphite particles according to claim 1, andone or more types of materials selected from the group consisting ofnatural graphite, artificial graphite, carbonaceous material-coatedgraphite, resin-coated graphite and amorphous carbon.
 6. A nonaqueoussecondary battery provided with a positive electrode, negativeelectrode, and an electrolytic solution; wherein, the negative electrodeis provided with a current collector and a negative electrode activematerial formed on the current collector, and the negative electrodeactive material contains the composite graphite particles according toclaim 1.